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FB/EK HOLDEN
350 HOLLEY CARBURETTOR
ENTHUSIASTS GUIDE
REVISION DATE UPDATE
0 November 2011 Initial draft for review.
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Table of Contents
1 Background .................................................................................................................................................................... 3 2 Operation ....................................................................................................................................................................... 5
2.1 Fuel Inlet System ...................................................................................................................................................... 5
2.2 Idle System ............................................................................................................................................................... 6
2.3 Main Metering System............................................................................................................................................... 7
2.4 Accelerator Pump System ......................................................................................................................................... 8
2.5 Power Enrichment System ........................................................................................................................................ 8
2.6 Choke System ........................................................................................................................................................... 9
3 Capacity (CFM) ............................................................................................................................................................ 10
4 Mounting ...................................................................................................................................................................... 12
4.1 Manifolds ................................................................................................................................................................. 12
4.2 Adaptor Plates......................................................................................................................................................... 14
4.3 Accelerator Linkage to Cable Modification .............................................................................................................. 14
4.4 Fuel, Vacuum and Choke ........................................................................................................................................ 16
5 Factory Specifications .................................................................................................................................................. 20
6 Assembly Diagram ....................................................................................................................................................... 21
7 Channels and Passages .............................................................................................................................................. 23
7.1 Metering Block (Float Bowl Side) ............................................................................................................................ 23
7.2 Metering Block (Main Body Side) ............................................................................................................................ 23
7.3 Main Body (Metering Block Side) ............................................................................................................................ 24
7.4 Main Body (Throttle Body Side) ............................................................................................................................... 24
7.5 Main Body (Choke Horn Side) ................................................................................................................................. 25
7.6 Throttle Body (Main Body Side) ............................................................................................................................... 25
7.7 Throttle Body (Manifold Side) .................................................................................................................................. 26
8 Disassembly and Overhaul Process ............................................................................................................................ 28
8.1 Kit Contents and Pre-disassembly .......................................................................................................................... 28
8.2 Special Tools........................................................................................................................................................... 30
8.3 Removal and Disassembly ...................................................................................................................................... 30
8.4 Cleaning and Inspection .......................................................................................................................................... 34
8.5 Assembly ................................................................................................................................................................ 35
9 Tuning.......................................................................................................................................................................... 39
9.1 Fuel Level ............................................................................................................................................................... 39
9.2 Idle Speed and Idle Mixture ..................................................................................................................................... 41
9.3 Fast Idle Speed ....................................................................................................................................................... 43
9.5 Main Metering Jets .................................................................................................................................................. 47
9.6 Power Valves .......................................................................................................................................................... 49
9.7 Venturi Sleeves ....................................................................................................................................................... 51
10 Troubleshooting ...................................................................................................................................................... 53
11 Modification ............................................................................................................................................................. 54
11.1 Fuel Supply Stability ................................................................................................................................................ 54
11.1.1 Wedged Float ..................................................................................................................................................... 54
11.1.2 Float Bowl Vent Baffle (Whistle) ......................................................................................................................... 54
11.2 Higher Air Flow ........................................................................................................................................................ 55
11.2.1 Choke Horn Removal ......................................................................................................................................... 55
11.2.2 K&N Stubstack ................................................................................................................................................... 56
11.3 Automatic Choke ..................................................................................................................................................... 56
11.3.1 Automatic Choke Operation ................................................................................................................................ 56
11.3.2 Electric Choke Conversion ................................................................................................................................. 59
11.3.3 Hot Air Choke Conversion .................................................................................................................................. 61
11.3.4 Automatic Choke Tuning .................................................................................................................................... 62
11.4 Power Valve Blowout Preventer (Check Ball). ......................................................................................................... 64
11.5 Better Fuel Metering (Adjustable Metering Block) ................................................................................................... 64
12 Contacts .................................................................................................................................................................. 66
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1 Background
This document aims to provide some information regarding fitment of
350 Holley carburettors to FB and EK Holdens. It contains:
historical information, such as which jets and bleeds were originally fitted to 350 Holley carburettors,
practical information on identification, disassembly and reassembly of 350 Holley carburettors, and
guidance on tuning, replacement parts and overhaul techniques.
It contains answers to many of the questions that seem to come up
routinely on most of the early Holden forums:
“What jets should I run in my early Holden?”
“Why is my Holley carburettor running so poorly?”
“How do I set up a cable throttle?”
Whilst this document is primarily related to the FB and EK Holdens, much of the information is applicable
to other early Holdens. Please bear in mind that the 350 Holley carburettor was not an original fitment to
early Holdens, and hence that limited documentation is known to exist. Much of the information below is
drawn from internet forums, discussion with enthusiasts and common sense. I have used photos and
other information from a wide variety of sources, particularly from the forums – if anyone is offended by
my use of the material, feels I have breached copyright or needs recognition, please let me know and I
will correct the issue immediately.
I have drawn information from the following sources:
The Holley 2300 Handbook by Mike Ulrich (most notably the drawings used in Section 2),
Super Tuning and Modifying Holley Carburettors by Dave Emanuel,
Some very good info on how automatic chokes work and are tuned from
http://www.chevelles.com/techref/Adjusting_Automatic_Chokes.htm
Some info published online by Holley at www.holley.com.
Equally, I have made opinions and drawn conclusions on some of the information I have found and
equipment I have owned, and have cross-referenced some material - if anyone believes that I have made
an error (or knows a better way to do something), please let me know and I will update the document...
after all, the main purpose here is to help other early Holden enthusiasts. I have marked some text in red
in this document where I am missing information – any help in closing these gaps is appreciated.
Like all things automotive, installing, operating and maintaining a carburettor comes with a risk. Leaking
fuel lines can lead to fires, jammed throttles can lead to out-of-control vehicles and items dropped down a
carburettor throat can cause massive engine damage (amongst other hazards). Any advice contained in
this document is to be taken at the reader’s risk – qualified mechanics should be consulted where
appropriate.
The 350 Holley is a common choice for Holden inline six-cylinder engines. Whilst the carburettor is oversized for the original FB/EK Holden grey motor, it is a good match for the larger displacement red motors. 350 Holley carburettors are also mandated in some forms of racing. For example, the current specification for both Australian Speedway Production Sedan and Modified Production Sedan classes
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mandates them for cars originally built in multiple carburettor or four-barrel carburettor form (though allows the venturi to be milled out from the stock 1
3/16” to 1
13/64”).
Having said that, Holley carburettors have a poor reputation amongst Holden owners. Some of the causes for this are:
Poor carburetor condition. Unfortunately, the average Holley has not been looked after too well, with a rebuild (either by a professional or by using a rebuild kit) strongly recommended,
Poor fuel atomisation, which can be resolved by installing venturi sleeves to increase fuel velocity, and
Poor fuel consumption, often caused by “over jetting” to try to hunt down the causes of the two issues above.
Each of the issues above will be tackled in this document. Holley has made a total of thirteen 350 CFM carburettors (all with 1
3/16” diameter venturis), as per the
table below:
List number Model
R3660 2300
R4055-1 2300
R4056-1 2300
R4144-1 2300
R4670 2300
R4791 2300
R4792 2300
R7448 2300
R80120 2305
R80320-1 2300
R80787-1 2300
R82010 2010
R87448 2300
However, most Holley 350 CFM
carburettors found on early Holdens are
List number R-7448, as per the image
to the right. I will focus on these
carburettors, and will refer to them as
“350 Holleys” for the remainder of the
document.
350 Holleys are a Model 2300 carburettor. Model 2300’s have been made by Holley since the mid 1950’s,
where they were used on Ford passenger car V8 engines. The 350 Holley is a non-staged two-barrel
carburettor (where the two barrels open at the same time by a common throttle shaft). 350 Holleys can be
identified by the List number, which is stamped onto the choke housing (see the red circle on the picture
above). Below the list number will be four digits (e.g. 1662). The first three digits are the day the
carburettor was manufactured (in this example the 166th day of the year) and the last digit is the year (2
for 1972).
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2 Operation
The 350 Holley carburettor has six basic systems that work together to provide the correct fuel/air mixture
over different engine loads:
• The fuel inlet system, which keeps a consistent level of liquid fuel “ready to go” in the carburettor,
• The idle system, which controls the fuel/air mixture at no-throttle and slight-throttle operation,
• The main metering system, which controls the fuel/air mixture at mid-throttle (or “cruise”) operation,
• The accelerating pump system, which adds a small “shot” of fuel when you initially put your foot down,
• The power enrichment system, which controls the fuel/air mixture at heavy throttle (hills, towing or
race) operation, and
• The choke system, which controls the air/fuel mixture for cold starting and warm-up.
Each of these systems will be described below.
2.1 Fuel Inlet System
The 350 Holley fuel inlet system consists of a fuel bowl, fuel inlet fitting, fuel inlet needle and seat, and a
float assembly. Fuel from the fuel tank is fed via the fuel pump to the carburettor. A sintered bronze filter
is usually installed in the fuel inlet fitting to capture dirt and rust and prevent them from blocking the fine
passages inside the carburettor. If the bronze filter (and associated spring and gasket) are omitted, an in-
line filter must be used. If the fuel
level is too low, the float (basically
a hollow brass or plastic ball that
floats on the fuel in the fuel bowl)
drops down and opens the fuel
inlet valve. This allows the
pressurised fuel to enter the
carburettor and begin filling the
float chamber. Once the fuel level
is high enough, the float rises, and
closes off the inlet valve. The float
chamber is vented by an internal
vent tube to the air horn. This
balanced pressure ensures that
fuel/air mixtures stay constant
even if the air filter is blocked by
dirt. The level of fuel in the float
chamber is adjusted by turning the
adjustment nut and lockscrew on
top of the float chamber (not shown in the simplified diagram). No disassembly is required to make this
adjustment, unlike the original factory Stromberg carburetors which required the air horn to be removed.
The fuel bowl on the 350 Holley carburettor is of the center-pivot type. This type of float is best for
speedway, gymkhana or road racing where fuel sloshing is from side-to-side (an aftermarket wedged-
shaped can also assist and will be discussed below). In drag racing applications, the front-to-back
sloshing of fuel (and lifting of the nose of the car) can cause this type of float to not operate as effectively.
A bumper spring under the hinge pin of the float helps to smooth out the float operation under stop/start
operation.
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Note that there are three types of float construction: hollow brass (left image below), solid black nitrophyl
(centre image below) and hollow white Duracon (right image below – the Duracon goes “pink” with use).
Brass floats are suitable for many different fuel types, but are unsuitable for blow-through forced induction
systems (as the increased pressure may crush the float) or where dual-fuel is used (as the LPG flowing
through the carburettor draws a substantive vacuum on the float bowl… and with the fuel inlet valve shut
that vacuum can easily crush the float). Solid nitrophyl floats are used when there is a risk of crushing,
though are not resistant to alcohol. Duracon floats are used as the factory-supplied float for new Holley
carburettors, and may be susceptible to both crushing and alcohol.
2.2 Idle System
Under very low engine speeds (idling), the engine does not produce enough vacuum to suck sufficient
fuel from the main metering system
(due to the near-closed throttle
plate). However, under the throttle
plate a high vacuum exists. This
vacuum is used to pull fuel from the
idle system. Fuel from the fuel bowl
enters the main wells through the
main metering jets that are screwed
into the metering block. Some of
this fuel is then bled off to an idle
well. The amount which is bled off
is limited by the idle feed restriction.
The idle fuel is then mixed with air
from the idle air bleed hole. The
idle air bleed hole also determines
when the idle system starts flowing
fuel – the larger the idle air bleed,
the slower the idle system is to start
flowing. The air/fuel mixture then
passes to the idle discharge port
below the throttle plate where it is
discharged, as per the diagram to
the right. Idle mixture screws are located on the sides of the primary metering block. These control the
volume of the pre-mixed air/fuel coming through the idle well. Turning the screws clockwise (in) will “lean”
the idle system, whilst turning the screws counterclockwise (out) will “richen” the idle system. This part of
the idle system is often referred to as “curb idle".
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The idle system also has a branch
that delivers air/fuel mixture to a
transfer slot above the throttle
plate. When the throttle is closed
(curb idle), there is little vacuum
above the throttle plate, and the
transfer slot does not flow. As the
throttle begins to open, the
transfer slot is uncovered, and
vacuum draws fuel from it, as per
the diagram to the left (the
diagram looks like a second idle
discharge hole, but it really is a
vertical slot). The transfer slot
provides fuel supply for the
transition between curb idle and
cruise (when the main metering
jets take over). The more the
throttle plate opens, the more of
the transfer slot is exposed to
vacuum, and the more fuel flows
through the slot. Note that the
fuel/air mixture flowing to the transfer slot is not altered by the idle mixture screws – the idle mixture
screws only adjust the curb idle mixture.
2.3 Main Metering System
The main metering system is
designed to supply the leanest fuel
mixture for cruising in the 35mph
(60km/h) and over range. Fuel
from the float bowl passes through
the main metering jets and enters
the main well. Here it is mixed with
air from the main air bleed. The air
emulsifies the fuel to allow easier
vapourisation, and lowers the
mixture viscosity for earlier feeding
of the main metering system. The
main air bleed hole also
determines when the main
metering system starts flowing fuel
– the larger the main air bleed, the
slower the main metering system
is to start flowing. Engine vacuum
pulls this air/fuel mixture and discharges it through the booster venturi. The booster venturi is located just
above the main venturi, and acts amplify the vacuum applied to the main metering and power enrichment
systems.
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2.4 Accelerator Pump System
The accelerator pump system injects a small amount of fuel into the carburettor throat when the throttle is
opened. The system provides
good throttle response (sharp
acceleration). When the throttle
turns, a pump cam acts against a
series of levers to move the pump
diaphragm. The pump cam profile
determines how fast and how
much fuel is injected for each
degree of throttle shaft rotation.
The pump diaphragm moves
upwards, closing the pump inlet
check valve and opening the
discharge check valve. Fuel is
forced through a discharge nozzle
and into the carburettor throat,
hitting the outside of the booster
venturi. When the throttle is
released, the diaphragm moves
back downwards under pressure
of the return spring. The
discharge check valve shuts, the pump inlet check valve opens and fuel is drawn from the float bowl to
refill the pump, ready for the next “shot”.
2.5 Power Enrichment System
When running under heavy load
(high speed, towing, travelling up
hills or racing), a richer mixture is
required, which is supplied by the
power enrichment system. 350
Holley carburetors utilize a
vacuum operated power
enrichment system. Manifold
vacuum is connected to the power
valve and holds the power valve
piston shut. Under heavy load, the
manifold vacuum decreases.
When the manifold vacuum is low
enough, it can no longer hold the
power valve piston shut. The
opening power valve supplies
extra fuel through the power valve
channel restriction to the main
metering system.
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The vacuum level at which the valve opens can be tuned by selecting different power valves.
2.6 Choke System
When starting a cold engine, a
richer than normal mixture is
required (because the slowly-
spinning engine produces little
vacuum to draw out fuel, and much
of the fuel condenses on the cold
inlet manifold walls). To do this, the
choke valve is shut, restricting air
into the carburettor. 350 Holley
carburettors are fitted with manual
chokes. A bowden cable operates
the choke linkage, opening and
closing the choke plate. The choke
linkage also incorporates a fast idle
cam. The fast idle cam bumps
open the throttle a small amount
when the choke is opened,
increasing engine speed. The
choke plate is offset and spring
loaded, such that the plate opens
slightly as the airflow increases
(leaning the mixture).
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3 Capacity (CFM)
There are some engine combinations where the original single-barrel Stromberg carburettor becomes
restrictive. It is common practice for enthusiasts to go hunting for a larger carburettor in the search for
more power… and sometimes that hunt finishes with a 350 Holley carburettor. Carburettors are often
rated in terms of the amount of fuel/air mixture they can flow at a given manifold vacuum. The flow rate is
expressed in cubic feet per minute, or CFM. Care needs to be taken though in that a given carburettor
may have several different venturi sizes, and hence several different flowrates (for example the BXUV-2
carburettor was offered in both 1/32” and
3/32” venturi sizes for early Holdens). The manifold vacuum used
to measure flow rate also varies. Some early published ratings for 1-barrel (e.g. B–Model Stromberg) and
2-barrel (e.g. WW-Model Stromberg and 350 Holley) carburetors were measured at 3” Hg. 4-barrel
carburettors (for example Holley 4150 carburettors) were rated at 1½”Hg. This means that a 600CFM 2-
barrel does not flow the same as a 600CFM 4-barrel – the 4-barrel flows 40% more as it is tested at
higher pressure drop.
The table below has been compiled from information on multiple websites. I have converted the
Quadrajet, Weber, and SU values to 3”Hg (they were published at 1.5”Hg). I have taken a single
published figure for Stromberg BXOV-2 carburetors (210CFM) and converted to the smaller BXOV-1 and
BXUV-2 carburettors by calculation based on the venturi and throttle bore diameters. The upshot of the
above is that the table below is very approximate, but should give some indication of the relative flowrate
achievable with different carburettors.
Carburettor Barrels Venturi diameter Flowrate (CFM @3”Hg)
Weber 38-DGAS 2 36mm/36mm 600
Rochester Quadrajet 4 2¼ “/1.35” 530
Mikuni 44 PHH 2 37mm/30mm choke 422
Holley 7448 (“350 Holley”) 2 13/16”/1
3/16” 350
SU HIF6 1 Variable 339
Weber 28/36-DCD 2 26mm/27mm 317
SU HS6 1 Variable 297
Stromberg BOV-2 (the “big brother swap”)
1 19/32” 287
WW Stromberg 2 128
/32”/ 128
/32” 280
Weber 32/34-DMTL 2 26mm/27mm 274
Weber 32/36-DGV 2 26mm/27mm 270
Weber 32/36-DGV 2 23mm/27mm 235
Stromberg BXV-2 1 15/32” 210
Stromberg BXUV-2 1 13/32” 201
SU HS4 1 Variable 201
SU H4 1 Variable 188
Holley EGC 2 11/16”/ 1
1/16” 185
Stromberg 48 2 11/32”/1
1/32” 175
Stromberg BXOV-1 1 13/32” 162
Holley 94/8ba 2 15
/16”/ 15
/16” 162
Stromberg LZ 2 1”/1” 160
SU H2 1 Variable 156
Holley 94/59 2 15
/16”/ 15
/16” 155
Stromberg 97 2 31
/32”/ 31
/32” 150
Holley 92 2 7/8”/
7/8” 142
Stromberg 81 2 13
/16”/ 13
/16” 135
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Note that the chart below indicates the required carburettor capacity for typical early Holdens (around
80% volumetric efficiency). The blue line shows that a 350 Holley has sufficient capacity for even a 202ci
motor running at 7500RPM.
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4 Mounting
4.1 Manifolds
A number of manifolds are available to mount 350 Holley carburettors to early Holden motors. One of the most common manifolds used for early Holdens is the Redline Performance Torker II 2V manifold (part number 12-65M – see image to the right). This is a multi-fit manifold that can be used on 149-202 red motors and 2.85 or 3.3 blue motors.
Redline Performance also produced a Roadmaster manifold (see image to the right). John Cain produced similar style manifolds for fitting 350 Holley carburettors to Holden red motors (see image to the right). AussieSpeed
® once
manufactured Cain Manifolds, though the name was later dropped from their product line. Lynx made a number of manifolds for fitting 350 Holley carburettors to Holden red motors, both in a style similar to the Redline and Cain manifolds (see image to the right), and in a long-runner format (see image to the far right. Unfortunately, Lynx are no longer producing manifolds. Firestreak made a water-heated manifold for 350 Holley carburettors. A manifold was also produced with SS cast into it.
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Manifolds for fitting 350 Holley carburettors to early Holdens are also made by AussieSpeed®. In 2009,
negotiations started with Kit Cullan to buy the Holden 6 cylinder Cullan Special manifolds to suit red and blue motor 6 cylinder engines. After testing and looking at the changes and benefits in the newer style of inlet manifolds AussieSpeed
® had been working on, the Ultraflow pattern equipment underwent massive
modifications and the Ultraflow name is no longer used. The following manifolds were made by AussieSpeed
®, but are no longer in production:
AussieSpeed
® part number AS0001 Holden 9-port 149-202 red motor.
This manifold will work on a standard engine. Street and competition manifold with tall plenum, plenum divider, long sweeping runners for wider rev range, this is a smaller runner designed for maximum air speed, good torque and fast acceleration. AussieSpeed
® part number AS0266 Cullen Special (Kit Cullan)
Ultraflow Holden 9-port 179-202 and 208/218 stroker engines. The corresponsinf 12-port manifold is part number AS0267. The current AussieSpeed
® manifold for red motor 9-port manifolds is part number AS0167 (below left)
and AS0169 for 12-port heads. The design offers high port velocity and a divided plenum that feeds all runners equally.
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4.2 Adaptor Plates
Whilst not ideal, it is also possible to reuse the factory inlet manifold, using an adaptor plate to mount the Holley carburetor flange (see dimensions below for various early Holden flanges).
350 Holley Bendix Stromberg BX-Model Bendix Stromberg WW-Model The factory single-barrel Holden Stromberg manifolds are able to be used with an adaptor plate (e.g. Redline Performance part No. 10-501 for 149-186 engines, 10-502 for 202 or, 10-503 for late 202). Whilst operable, this setup has poor airflow, reducing performance. They suffer from fuel cone breakup which will allow the fuel to fallout and puddle rather than moving smoothly through the intake system. WW Stromberg manifolds are able to be used with a similar adaptor plate (e.g. Redline Performance part No. 10-187 or 10-233). The open nature of this plate has a far less disruptive effect on fuel flow.
For Varajet II manifolds, an adaptor plate is also available (e.g Redline Performance part Nº. 10-219). Varajet carburetors were found on WB Holden 202, UC Torana 1.9L, VC and VH Commodore 1.9L, 2.85 and 3.3L, and some VK Commodore 3.3L engines (some VK Commodores had Bosch LEII-Jetronic fuel-injection).
4.3 Accelerator Linkage to Cable Modification
With some carburettor manifolds and linkages, it is possible to use the original FB/EK Holden throttle
linkage (the swinging bar type) with a little bending. However, the 350 Holley carburettor is generally
operated by converting the throttle linkage to a cable type, eliminating the complex linkage. A number of
pedal/cable assemblies can be mounted into FB/EK Holdens, notably HZ Holden and Commodore.
A neat (and simple) solution is to retain the
original FB/EK pedal, and modify it to suit the
cable from a Mitsubishi L300 Express van.
These vehicles were sold from 1980-1986,
and look similar to the photographs to the
right. Note that the later models however do
not have the required clevis at the cable end.
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The Mitsubishi L300 accelerator cable is quite long, and can be shortened with a simple pair of
sidecutters to the correct length once installed.
To undertake the conversion:
1. Remove all the throttle linkage except the pedal.
Remove the clip connecting the lower cross shaft operating rod to the accelerator pedal (under the
car),
Unbolt the lower cross shaft assembly (four phillips-head bolts located under the car).
Remove the clip connecting the upper cross shaft operating rod to the upper cross shaft assembly.
All the linkage from under the car should now fall out
Unbolt the upper cross shaft support (two phillips-head bolts per bracket, one bracket on drivers
and passengers side of firewall.
Disconnect the throttle control upper rod from the carburettor. All the linkage from in the engine bay
should now fall out.
Don’t discard all the parts yet – the upper cross shaft support from the passenger’s side makes a good
bracket for supporting the cable later.
2. Attach the Mitsubishi L300 accelerator cable clevis to the original Holden accelerator pedal, using the
hole that the lower cross shaft operating rod mounted to (under the car). The cable can be attached
with a pin and split pin, or by using a small bolt and nylock nut (do not overtighten the nut as it will bind
the clevis).
3. The cable will now run into the cabin using the Mitsubishi L300 cable guide. You will need to drill a
hole in the floorpan for the cable to pass through, and another two for the cable guide mounting bolts.
Mount the cable guide using nuts, bolts and spring washers, with some sealant under the cable guide
to prevent water ingress to the cabin. The photographs above show the mounting of the clevis and
cable guide on a number of vehicles.
4. Run the cable inside the cabin, up the firewall (under the carpet/floor mat) and pass it out through the
grommet where the original choke cable passes through. The picture to the right
shows the cable routing with the carpet/floor mat removed.
5. The cable then passes across the engine bay to the carburettor throttle linkage. The
cable must be mounted, similarly to the way that the choke tube holder assembly
mounts the original choke cable (the photograph below to the right shows a holder
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assembly fitted to a Holley carburettor, and the photograph to the left to a twin Stromberg setup).
6. The cable setup often feels much lighter than the original throttle linkage, and an extra (or heavier)
return spring can assist in returning the pedal feel. The photo below to the right shows a return spring
mounted off a bracket on the original battery tray.
7. The cable assembly should be checked and adjusted so that the carburettor both achieves wide open
throttle, and returns to idle. It’s a good idea not to cut the cable to final length until this has been done.
In some cases, it may be necessary to extend the accelerator pedal lever (by welding on a piece of flat
bar) in order to get enough pedal travel to attain full throttle.
4.4 Fuel, Vacuum and Choke
The fuel connection at the inlet of the Holley 350 carburettor is located at a similar position to the original Stromberg carburetor for FB/EK Holdens. Provided the manifold chosen does not have long runners, it is possible with some gentle bending to get the original fuel line to align with the carburettor. The Holley carburettor inlet is AN-5 (SAE thread size ½-20) thread, as is the original FB/EK fuel inlet line. Note that the recommended fuel pressure for 350 Holley carburettors is 5-7 psi. Whilst standard GMH grey/red/blue motor fuel pumps (at 3.9 - 4½ psi) are adequate, care must be taken when an electric fuel pump has been added – the higher than required fuel pressure forces open the needle and seat, flooding the engine. The chart and table below provide some guidance. When using inline fuel pumps (notably Holley), a pressure regulator is mandatory to prevent flooding.
Fuel Pump Maximum Pressure (psi) Free Flow (GPH) Facet SS208 3½ 14
Facet SS171 3½ 14
Later Holden (blue motor steel can) 3.9 9½
Facet SS500 4 25
Facet 60104 4 25
Facet IP002 4 32
Early Holden (grey/red glass bowl) 4½ 9
Facet SS148 4½ 24
Facet SS501 4½ 30
Facet SS165 5 15
Carter GP4600HP 5 100
Facet STS504 5½ 30
Facet IP007 5½ 36
Facet IP131 5½ 36
Facet IP220 5½ 36
Facet 60106 6 32
Facet SS135 6 34
Carter GP4603HD 6 43
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Carter GP4070 6 72
Facet SS502 7 32
Facet STC505 7 35
Facet IP051 8 30
Facet RTW506 8 40
Facet BTP001 8 40
Facet BTP001 8 40
Carter GP4594, GP4389, GP4259 and GP4602RV 8 72
Facet SS200 9 32
Facet SS503 10 34
Facet 60107 10 34
Holley Red 10 100
Facet SS185 11½ 29
Facet 40222 11½ 33
Facet 40223 11½ 33
Facet 40237 11½ 33
Carter GP4601HP 18 100
Holley Blue 18 110
Holley Black 18 145
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0
20
40
60
80
100
120
140
160
0 2 4 6 8 10 12 14 16 18
Flo
w (
GP
H)
Prressure (psi)
Holley red
Holley blue
Holley black
Stromberg inlet pressure
350 Holley inlet pressure
early Holden fuel demand
Pressure regulator required to drop Holley red pressure down to 350 Holley 3-7psi inlet pressure range.
Page 19 of 66
Increasing fuel pressure will also require the fuel level in the carburettor to be reset – as a rule of thumb, every one psi of fuel pressure increase will raise the fuel level
1/32”.
The 350 Holley carburettor has a number of vacuum sources:
full manifold vacuum, a 3/8” pipe located behind the throttle body provides vacuum for power brakes,
vacuum windshield washers and PCV valves (if you have more than one of these, you will need to use plastic vacuum tees from an automotive parts store like SuperCheap or Repco). Full manifold vacuum is tapped off from below the throttle plates. You get more manifold vacuum when you take your foot off the throttle (this is why pre-EK vacuum wipers work so well when you lift your foot, but run poorly when you have your boot into it driving uphill in the pouring rain!).
timed spark vacuum, a 3/16” pipe located in the choke side of the primary metering block. Timed-spark
vacuum (sometimes referred to as distributor vacuum) is taken from above the throttle plates. Timed spark vacuum is exactly the same as manifold vacuum – except that it is shut off under zero throttle (i.e. under idle conditions, there is huge manifold vacuum, but zero distributor vacuum). The strategy behind distributor vacuum (generally used in later-model carburettors) is to remove vacuum advance at idle, causing the vehicle to run hotter and combust exhaust emissions (often with the help of air injection systems at the exhaust manifold).
Early Holdens were designed to run timed spark vacuum (the vacuum port connection is at the throttle body above the throttle plate – see diagram to the right). There is no harm in running distributor vacuum. However, for cars with large cams (high valve overlap and poor vacuum), tapping into manifold vacuum (and blocking off the distributor vacuum ports on both the carburettors) can give better vacuum signal at idle, more advance and hence better idling. This can also reduce engine temperature at idle. Note that the original FB/EK vacuum line ends in a
1/8” NPT thread nipple (the nut is
7/16” AF). If using the full
manifold connection, rubber vacuum hose can be clamped over both the original nipple and the 350 Holley manifold vacuum pipe. If the timed spark vacuum connection is used, it may be necessary to cut off the original nipple and use smaller diameter rubber vacuum hose to clamp to the (cleaned and smoothed) pipe end. Cap-off any vacuum source that is not used (plastic vacuum caps are available from automotive spare parts stores like SuperCheap and Repco from the same carousel that sells small blister packs of nuts and bolts). 350 Holley carburettors were originally fitted with manual chokes, which will connect directly to the original FB/EK choke cable.
Page 20 of 66
5 Factory Specifications
The following table lists the factory (“out of the box”) specifications for the List number 7448 350 Holley
carburettor:
Model number 2300, List number 7448.
350CFM (@ 3”Hg) capacity. Intended as stock performance replacement for 2-barrel street applications. Also mandated carburettor for some oval track racing sanctioning bodies.
Venturi diameter 13/16” (main body part number 6R1919).
throttle bore 1½” (throttle body part number 12R-5174B, also stamped 7448 underneath). Throttle plates stamped 107. Throttle bore and shaft assembly part number 12R11070A.
Manual choke.
30cc accelerator pump (accelerator pump cover part number 34R2178B) with “Orange” accelerator pump cam (part number 41R466) in position #2. Tube type discharge nozzle size 0.031” (part number 121-31).
Viton® tipped adjustable needle with 0.110" seat (part number 6-504).
#60 main metering jets with ¼-32 UNF thread (part number 122-61).
Fuel bowl part number 134-103 (marked 36R4649B), with fuel bowl gasket part number 108-83-2. Centre hung float part number 116-2. Fuel inlet fitting tapped to AN-5 (SAE thread size ½-20) to suit 5/16” OD tube.
Metering block part number 134-203 with metering block gasket part number 108-89-2. Metering blocks stamped L7448
Single stage normal flow power valve opening at 8.5”Hg (part number 125-85). Power valve channel restriction approximately 0.056”.
One 3/16” timed-spark vacuum port (tapped from left side of metering block) and one
3/8” manifold
vacuum port (tapped from rear of throttle body). Choke vacuum port (for fitment of hot-air or electric choke) controlled by a 0.055” (#54 drill) orifice screwed into the base of the throttle body.
Ford automatic transmission kickdown, does not work with automatic overdrive transmissions.
Renew Kit part number 37-1536, Trick Kit part number 37-933, Fast Kit part number 37-1543.
WARNING: If you are using this carburetor with a GM overdrive transmission TH700R4 or a TH200R4, you must use a transmission kickdown cable bracket (Holley P/N 20-95) and stud (Holley part number 20-40). Otherwise, SEVERE transmission damage WILL result. This carburetor is not designed to work with ANY other automatic overdrive transmission.
Page 21 of 66
6 Assembly Diagram
Holley provides a generic assembly diagram for Model 2300 carburettors at
http://www.holley.com/data/TechService/Technical/2300%20Exploded%20View.pdf, and for Model
2300 carburettors set up for 3x2 configuration at
http://www.holley.com/data/TechService/Technical/2300%20(3x2)%20Exploded%20View.pdf.
However, neither diagram maps out the 350 Holley carburettor very simply – both diagrams are generic,
and have either extra or missing parts. I have taken both these diagrams and “cut and shut” them to
make the 350 Holley assembly diagram below.
Page 22 of 66
No Description No Description No Description
1 Fuel bowl screw (4 off) 29 Metering body 57 Back-up plate stud nut
lockwasher
2 Bowl screw gasket (4 of) 30 Metering body gasket 58 Stud nut
3 Fuel inlet fitting 31 Power valve gasket 59 Throttle body
4 Inlet filter gasket 32 Power valve 60 Pump cam lock screw
5 Inlet filter screen 33 Throttle body gasket 61 Pump cam
6 Inlet filter spring 34 Main body 62 Curb idle screw spring
7 Inlet fitting gasket 35 Pump discharge nozzle
screw 63 Curb idle screw
8 Fuel inlet check plug 36 Discharge nozzle gasket 64 Throttle shaft bearing
9 Check plug gasket 37 Pump discharge nozzle 65 Throttle shaft centre bearing
10 Accelerator pump cover
screw (4 off) 38 Discharge nozzle gasket 66 Throttle shaft bearing
11 Accelerator pump cover 39 Pump discharge check valve 67 Fast idle cam lever screw
12 Pump diaphragm 40 Choke plate screw (2 off) 68 Fast idle pick-up lever
13 Diaphragm return spring 41 Choke plate 69 Fast idle cam lever spring
14 Float lever shaft 42 Choke shaft and lever 70 Fast idle cam lever
15 Float assembly 43 Choke link seal 71 Throttle plate screw
16 Float spring 44 Choke link 72 Throttle plate (2 off)
17 Float shaft retainer 45 Control lever nut 73 Fast idle cam lever spring
18 Fuel valve seat lockscrew 46 Lockwasher 74 Fast idle cam lever screw
19 Lockscrew gasket 47 Choke lever and swivel
assembly 75
Throttle body to main body screw (5 off)
20 Fuel valve seat adjustment
nut 48 Swivel screw 76 Choke vacuum supply orifice
21 Adjustment nut gasket 49 Fast idle cam plate 77 Pump operating lever
adjustment nut
22 Fuel valve assembly 50 Plunger spring 78 Pump operating lever
adjustment spring
23 Fuel bowl gasket 51 Fast idle cam plunger 79 Pump operating lever
adjustment screw
24 Fuel bowl assembly 52 Fast idle cam and shaft
assembly 80 Pump operating lever
25 Metering body gasket 53 Back-up plate and stud
assembly 81
Pump operating lever retainer
26 Idle needle (2 off) 54 Choke rod lever and bushing
assembly 82 Throttle shaft and lever
27 Idle needle seal (2 off) 55 Choke spring 83 Throttle return spring
28 Main metering jet (2 off) 56 Spring washer 84 Choke housing screw (3 off)
Page 23 of 66
7 Channels and Passages
The images below illustrate the channels and passages cast and machined into the 350 Holley
carburettor.
7.1 Metering Block (Float Bowl Side)
1. locating dowels
2. float bowl vent from float bowl to main body
3. accelerator pump discharge from float
bowl
4. main metering jet mounting holes
5. timed-spark vacuum port
6. power valve mounting hole
7. accelerator pump transfer passage
from fuel bowl to main body
8. curb idle fuel passage from needle
valve to main body
9. idle well
10. timed-spark vacuum passage
11. main well
12. gasket sealing bead
7.2 Metering Block (Main Body Side)
1. locating dowels
2. float bowl vent from float bowl to main body
3. curb idle discharge from needle valve to main body
4. idle transfer fuel to main body
5. timed-spark vacuum port
6. power valve mounting hole
7. accelerator pump transfer passage
from fuel bowl to main body
8. air bleed from main body to main
well (approximately 0.0385”
diameter)
9. air bleed from main body to main
well (approximately 0.027”
diameter)
10. timed spark vacuum port to nipple
11. idle feed to idle well
12. idle bleed air from main body
13. idle down well
14. main air well
Page 24 of 66
15. power valve channel restriction (approximately 0.056” diameter)
16. idle fuel from main well
17. main fuel passage from main well to main body
18. timed-spark vacuum passage
19. gasket sealing beads
7.3 Main Body (Metering Block Side)
1. fuel from accelerator pump to pump discharge nozzle
2. air from idle air bleed to metering block
3. fuel emulsion to main discharge nozzle
4. air from main air bleed to metering block
5. float bowl vent from float bowl to main body
6. vacuum from above throttle plate to timed-
spark port
7. fuel emulsion to curb idle discharge hole
8. fuel emulsion to idle transfer slot
9. locating dowels
10. fuel bowl vent to air cleaner
11. fuel bowl/metering body/main body mounting
holes
12. air from idle air bleed to metering block
13. not used (blanked hole)
14. power valve vacuum chamber
15. manifold vacuum from under throttle plates to
power valve vacuum chamber
7.4 Main Body (Throttle Body Side)
1. throttle body to main body mounting holes
2. choke vacuum supply for hot air and electric
chokes
3. idle fuel emulsion to idle discharge hole below
throttle plate
4. transfer idle fuel emulsion to transfer slot
above throttle plates
5. timed-spark vacuum connection to hole above
throttle plate on one barrel only
6. manifold vacuum from under throttle plates to
power valve
7. not used – blind channel
8. booster venturi
9. not used – blind hole
Page 25 of 66
10. not used – hole through to airhorn, but no hole in throttle plate or throttle plate gasket
7.5 Main Body (Choke Horn Side)
1. unused (blind holes)
2. air cleaner stud mounting hole
3. fuel bowl vent tube
4. venturi
5. booster venturi
6. unused (blind hole)
7. idle air bleed (approximately
0.035” diameter)
8. main air bleed (approximately
0.0775” (5/64”) diameter)
9. not used (blind holes)
10. fuel from accelerator pump to
pump discharge nozzle
7.6 Throttle Body (Main Body Side)
Page 26 of 66
1. throttle body to main body mounting holes
2. transfer idle fuel emulsion to transfer slot above throttle plates
3. idle fuel emulsion to idle discharge hole below throttle plates
4. timed-spark vacuum connection to hole above throttle plate on one barrel only
5. unused (blind hole)
6. manifold vacuum from under throttle plates to power valve
7. choke vacuum supply for hot air and electric chokes
8. manifold vacuum port
9. mounting holes for brackets (e.g. dashpot)
7.7 Throttle Body (Manifold Side)
1. throttle body to main body mounting holes
2. choke vacuum supply for hot air and electric chokes
3. manifold vacuum from manifold to nipple
Page 27 of 66
4. manifold vacuum from under throttle plates to power valve
5. mounting holes for brackets (e.g. throttle linkage or dashpot)
6. unused hole
7. mounting holes for brackets (e.g. throttle linkage or dashpot)
8. manifold vacuum port
Page 28 of 66
8 Disassembly and Overhaul Process
The following process describes the process of removal, disassembly and overhaul (often referred to as
“putting a kit through”) for a 350 Holley carburettor.
8.1 Kit Contents and Pre-disassembly
The 350 Holley carburettor overhaul illustrated below will be completed with a
genuine Holley Fast Kit, part number 37-1543. Suits R4412, 4412-1, 4412-2, 4412-3,
7448, 9647, 84412, 87448. The kit contains the following parts:
1. Metering body gasket
– see note 1.
2. Fuel bowl gasket –
see note 2.
3. Correct flange gasket.
4. Incorrect flange
gasket (not used for
overhaul of 350
Holley carburettors).
5. Paper gasket to seal
choke air supply.
6. Four plastic bowl
screw gaskets.
7. Rubber umbrella inlet
(not used for overhaul
of all 350 Holley
carburettors).
8. Two cork idle needle
seals.
9. Incorrect throttle body
gasket (not used for
overhaul of 350
Holley carburettors).
10. Correct throttle body
gasket.
11. Incorrect 50cc accelerator pump diaphragm (not used for overhaul of 350 Holley carburettors).
12. Correct 30cc accelerator pump diaphragm.
13. Fuel filter.
14. No. 65 power valve – see note 3.
15. Fuel valve assembly.
16. Inlet fitting gasket.
17. Power valve gasket – see note 4.
18. Fuel filter gasket.
19. Adjustment nut gasket.
Page 29 of 66
20. Check plug gasket.
21. Lock screw gasket.
22. Pump discharge screw gasket.
Plus two sheets of paper instructions.
Note 1: the metering body gasket supplied in the kit is slightly
different to the original Holley part. One hole is not stamped in
the kit gasket (see red circle on the image to the right).
However, this hole is not used in 350 Holley carburettors, and
the kit gasket is acceptable to use.
Note that 350 Holley carburettors use no
accelerator pump transfer tube. This
means that the gasket on the left (set up
for an accelerator pump transfer hole),
as supplied in the kit, must be used. Use
of a gasket like the one on the right will
lead to internal leaks and poor
operation.
Note 2: the fuel bowl gasket supplied in the kit has two
accelerator pump passages (the gasket is symmetrical). Some
gaskets however only have one passage (see image to the
right), and care must be taken that the gasket is put on the right
way around – otherwise the gasket will cover the accelerator
pump, and no pump shot will occur on acceleration.
Note 3: the factory 350 Holley power valve opens at 8.5”Hg
(often referred to as an “85” power valve). The kit above
(and most kits nowadays) supplies a valve that opens at
6.5”Hg (a “65 power valve). Power valves may be identified
either from the opening setting being stamped into one of
the valve nut flats, or onto the valve head as seen in the
image to the right (which shows a 65 power valve).
Note 4: two different power valves are available for 350 Holley carburettors.
The first (labeled “A” in the image to the right) has drilled holes in the valve,
and a tanged gasket. The second (labeled “B” in the image to the right) has
square “window” holes in the valve, and uses a circular gasket. The kit above
(and most kits nowadays) uses the second type. Care must be taken to use
the correct gaskets with the correct type power valve to avoid leakage.
Page 30 of 66
Prior to disassembling the carburettor, it is worthwhile checking for worn throttle shaft bearing areas. To
do so, start the engine and leave it idling with the air cleaner in place. Spray some WD40 around the
throttle body where the throttle shaft assembly passes through either side, using the red squirty straw on
the can of WD40 to get at the right area. Make sure there is no grease or dirt around the area that could
block the WD40 from getting to the throttle body. If the engine revs pick up, then the throttle shaft bearing
areas are worn (letting in WD40 under vacuum to fuel the motor) and should be professionally rebushed
during the rebuild.
8.2 Special Tools
Most of the overhaul process can be undertaken with basic garage tools – screwdrivers, long nosed
pliars, a set of imperial spanners and a 1” socket or spanner, a gasket scraper, some imperial drill bits
and a set of feeler gauges. Whilst they are not critical, if you are overhauling a few Holley carburettors it is
worthwhile buying a set of imperial feeler gauges and “narrow and bending” the 0.011”, 0.015” and 0.020”
gauges – more on these below. A torque wrench calibrated in inch-pounds (not foot-pounds!) is also
useful (not critical), though needs a slot-head screwdriver fitting (they are also useful for adjusting
automatic transmission bands during servicing if you need an excuse to buy one).
8.3 Removal and Disassembly
1. Remove the air filter, taking care not to drop the stud nut down the carburettor throat.
2. Allow the engine to cool prior to disconnecting the fuel line at the carburettor fitting. Note that the fuel
line may be under pressure from the fuel pump, and can leak some fuel – some rags to mop the fuel
up or a steel drift to plug rubber fuel lines are useful.
3. Disconnect the manifold vacuum hose at the rear of the throttle body base, and the timed spark
vacuum hose at the side of the metering block. Plug off the disconnected hoses. Note that in some
cases either or both of these hose connections may be not used, and may hence be capped off at
the carburettor with plastic caps.
4. Loosen the choke cable holder and swivel screw and disconnect the choke cable.
5. Remove any throttle return spring(s) fitted. Loosen any throttle cable clamp fitted then disconnect the
throttle cable (a throttle stud is often used).
6. Undo the four carburettor to manifold nuts and remove them, taking care not to drop them down the
carburettor throat. Lift the carburettor off the manifold studs, taking care not to bump any dirt down
the manifold. Remove the manifold gasket then cover the manifold opening with clean rag.
7. Undo the two slot-head screws and remove the choke cable holder bracket. Undo the remaining slot-
head screw and remove the choke lever and swivel assembly, fast idle cam plate and fast idle cam
and shaft assembly as one unit.
Page 31 of 66
8. Remove the choke link
hairpin clip then remove the
back-up plate and stud
assembly, choke rod lever
and bushing assembly and
choke spring as one unit.
Remove the paper gasket
covering the choke vacuum
passage (see red arrow on
image to the right). Some carburetors have this passage filled with a lead ball – if a lead ball is
present, do not disturb it.
9. Turn the carburettor over and undo the phillips-head fast idle
cam lever screw. Remove the fast idle pick-up lever, fast idle
cam lever spring and fast idle cam lever as one unit.
10. Undo the 11
/32”
control lever and
back-up plate stud
nuts, and unscrew
the slot head fast
idle cam lever
screw.
Dissassemble the
choke assemblies
Note that the choke link and choke link seal are not removed from the airhorn. To do so requires removal
of the choke plate. This is quite an involved process (filing off the choke plate screws and restaking them
on reassembly), and not generally needed for an overhaul.
11. To support the carburettor and prevent damage to the throttle
plates/throttle body face, fit some spare ½” AF bolts into the
flange holes. The bolts act as “legs”, supporting the carburettor
off the workbench. Whilst the carburettor is together enough to
get a good grip on it, loosen the fuel inlet fitting (1” AF) and fuel
valve seat lock screw and nut (slot-head/ 5/8” AF).
Page 32 of 66
12. Undo and remove the four slot-head fuel bowl screws. Separate
the fuel bowl from the metering body – if the gasket is glueing
the two together, a gentle tap with a screwdriver or hammer
handle may loosen it. Retain the old fuel bowl gasket to
compare to the new one from the kit.
13. Undo and remove the fuel inlet fitting (1” AF) and associated
gasket. Pull out the sintered bronze fuel filter, fuel filter gasket
and fuel filter spring (note that these are missing from the
picture to the right). Unscrew the fuel valve seat lock screw
(slot-head), remove the fuel valve seat adjustment nut and
unscrew the fuel valve assembly (5/16” AF). Remove the slot
head fuel level check plug and associated gasket. Pick out the
bowl screw gaskets (which have generally stuck to the fuel
bowl), taking care not to damage the fuel bowl faces.
14. Undo and remove the two slot-head float shaft retainer screws. Remove the float shaft retainer, float
lever shaft, float spring and float assembly as one unit. Take careful note of the relationship between
the float shaft retainer, float lever shaft, float spring and float assembly before disassembling (see
picture below right).
15. Undo and remove the four phillips-head accelerator pump cover screws. Remove the accelerator
pump cover, accelerator pump diaphragm and diaphragm return spring. The hanging ball non-return
valve and retainer (see picture below right) are not removed.
Page 33 of 66
Note that
late-
production
350 Holley
carburettors
used a
“plastic
umbrella”
non-return
valve. If one is fitted, remove and discard the plastic umbrella.
16. Separate the metering body from the main body – if the gasket
is glueing the two together, a gentle tap with a screwdriver or
hammer handle may loosen it. Retain the old metering body
gasket to compare to the new one from the kit.
17. Unscrew the two main metering jets with a wide-blade slot-head
screwdriver. Carefully remove the fuel bowl gasket, taking care
not to damage the metering block face. Unscrew the two slot-
head idle needle valves and pick out the associated cork seals.
Remove the power valve with a 1” AF socket. Carefully remove
the metering body gasket, taking care not to damage the
metering body face
18. Unscrew and remove the phillips-head
pump discharge nozzle screw. Remove the
pump discharge nozzle and associated
gasket. Turn the main body upside down
and catch the pump discharge needle valve
as it falls out.
Page 34 of 66
19. Unscrew and remove the five phillips-head throttle body to main body screws. Separate the main
body, throttle body and associated gasket. Remove the slot-head choke air bleed orifice from the
base of the throttle body.
20. Unscrew the slot-head pump cam lockscrew and remove the
lockscrew and pump cam. Undo and remove the accelerator
pump adjustment screw, nut (3/8” AF) and spring. Remove the
pump operating lever retainer and slide off the pump operating
lever. Unscrew the throttle stop screw and spring.
8.4 Cleaning and Inspection
1. Clean all parts in some petrol to remove most of the oil and dirt. Ensure good ventilation and no open
flames when washing parts with petrol (or any of the solvents below). An alternative is to use one of
the spray type “carburettor and throttle body cleaners” available from SuperCheap, Repco etc. Most
of the cleaners available are made for spraying down a carburettor throat with the engine running,
rather than detailed cleaning of a disassembled carburettor. They tend to be mainly solvent,
evaporate very quickly, and are this not much use for “soaking” parts. They are also not very suitable
for removing the carbon (“coke”) that builds up inside carburettors (what little they dissolve tends to
restick as the cleaner evaporates). From trying some of them, I personally believe these spray
cleaners are little (if any) better than using straight petrol for cleaning disassembled carburettors.
Many forums recommend the use of “dip” cleaners to soak parts in (for example Berrymans B9
Chem Dip, which has a number of solvents, cresols and sodium bichromate). Some hunting has
shown that “dip” cleaners are very hard to come by in Australia. One that is available is Yamalube
Carburettor Cleaner, though I have not tried it. Paint thinners also does a fair job of removing the
gunk. Note that the plastic choke link seal will be left in the airhorn – whilst it is tolerant of a short
bath in petrol or thinners, soaking it for an extended period is not advisable. The same goes for the
plastic pump cam.
2. Blow out all passages with compressed air in the opposite direction to normal flow. Do not rod-out
any jets or passages with drills or wires unless absolutely necessary as it is likely to change their
flow characteristics. If a compressor is not available, a bicycle pump (with a ball inflation needle
fitted) will do the task.
3. Use a steel rule to check that the main body assembly, metering body and float bowl are flat where
they join. Should any of these surfaces not be flat, replacement may be required. Whilst the main
body can be machined flat (within reason), the metering body and float bowls each have gasket
sealing beads. Machining these parts removes the beads, causing difficulty in gasket sealing.
Page 35 of 66
4. Check the idle discharge holes and transfer slots in the throttle body assembly to make sure they
have no carbon deposits.
5. Examine the idle needle valves. If they are ringed or grooved they must be replaced.
6. Inspect the main metering jets to ensure they are clean and unmarked.
7. Check the float assembly for dents and punctures (for example the float
on the left of the image is severely dented, either from being used in a
blow-through forced induction system, a severe backfire, poor handling
during reassembly or being used in a dual-fuel (LPG) vehicle).
8. Check the throttle lever and shaft assembly where it passes through either side of the throttle body
assembly (62) for looseness. Worn assemblies should be professionally rebushed during the rebuild.
Check that the throttle valve opens and closes correctly. Check that the throttle plate screws are tight
and staked.
9. Check the choke shaft assembly where it passes through either side of the air horn for looseness.
Check that the choke valve assembly opens and closes correctly and
that the choke plate screws are tight and staked..
10. With the fuel bowl inverted, check the clearance between the accelerator
pump check ball and the retainer bar. The clearance should be 0.011-
0.015”. Note that in order to do this a set of feeler gauges will need to be
modified (narrowed and bent…a handy hint is to bend them first then file
them narrow, as they don’t like being bent once they are narrowed). The
retainer bar can be bent gently, though care needs to be taken not to pry
the bar from its end fittings.
8.5 Assembly
When assembling the carburettor, the bolts and fittings may be torqued. Whilst not absolutely essential, torqueing to a set value can prevent stripping threads (most of the screws are into alloy), or uneven tightening (leading to leaks). The following torque settings should be applied during assembly. Note that the values are in inch-pounds (not foot-pounds!).
Application Fastener Size-Threads Per
Inch
Torque Range Minimum-Maximum
(inch-pounds)
Dry Oiled
Fuel bowl screws 12-24 25-30 19-22
Main metering jets ¼-32 30-40 20-30
Fuel valve seat lock screw ¼-32 50-60 40-45
Float shaft retainer screws 6-32 3-5 2-3
Fuel bowl inlet fitting 7/8-20 200-250 150-190
Power valve ½-28 40-50 30-38
Accelerator pump cover screws 8-32 6-10 5-8
Choke vacuum restrictor grub screw 10-32 10-15 8-11
Fuel level check plug 5/16-24 55-65 40-50
Choke housing screws 8-32 6-10 5-8
Pump discharge nozzle screw 12-28 25-30 19-22
13. Install the pump cam and slot-head pump cam lockscrew into position #2 of the throttle shaft.
Page 36 of 66
14. Install the accelerator pump adjustment screw, nut (3/8” AF) and spring to the pump operating lever,
leaving them loose for now.
15. Install the throttle stop screw and spring into the throttle body.
16. Fit and tighten the slot-head choke vacuum restrictor grub screw into the base of the throttle body.
17. Using the new throttle body gasket from the kit, fit together the main body and throttle body. Take
care that the correct gasket is chosen from the kit. Tighten the five phillips-head throttle body to main
body screws.
18. Fit some spare ½” AF bolts into the throttle body flange holes. The bolts act as “legs”, supporting the
carburettor off the workbench.
19. Install the pump discharge needle valve. Using the new gasket from the kit, install the pump
discharge nozzle and tighten the phillips-head pump discharge nozzle screw. Set aside the
throttle/main body assembly for now.
20. Screw the two main metering jets into the metering block with a wide-blade slot-head screwdriver.
21. Using new cork seals from the kit, install the two slot-head idle needle valves into the metering block.
Screw them in gently until they seat (do not overtighten!) then back them out 1½ turns.
22. Install the power valve and gasket (both from the kit) into the metering body with a 1” AF socket. Set
aside the metering block assembly for now.
23. If the pump non-return valve is the “plastic umbrella” type, install a new “umbrella”. Wet the umbrella
nipple with some spit, then insert the umbrella nipple from the outside of the fuel bowl and gently
pulling it through from the inside. You will feel the nipple “click” as it seats. Cut the top off the nipple,
leaving a small amount protruding into the fuel bowl (if the whole nipple is left it will interfere with the
float at low float level).
24. Place the new diaphragm return spring into the accelerator pump housing. Fit the accelerator pump
diaphragm from the kit over the spring, taking care to select the 30cc diaphragm from the kit. Fit the
accelerator pump cover then install and tighten the four phillips-head accelerator pump cover
screws.
25. Reassemble the float shaft retainer, float lever shaft, float spring and float assembly as one unit.
Install the assembly into the fuel bowl, tightening the two slot-head float shaft retainer screws.
26. Install the new fuel valve assembly from the kit, using the adjustment screw and locknut gaskets
from the kit. Install the fuel valve seat adjustment nut and the slot-head fuel valve seat lock screw.
27. Note that there are three types of float construction: hollow brass (left image below), solid black
nitrophyl (centre image below) and hollow white Duracon (right image below).
Page 37 of 66
For brass and nitrophyl floats, turn the fuel bowl upside down, then
adjust the fuel valve seat adjustment nut until the float sits in the middle
of the fuel bowl (as per the image to the right). Tighten the slot-head
fuel valve seat lock screw.
Duracon floats ride higher on the fuel than either the brass or nitrophyl
float and, therefore, a higher setting is in order. For Durcon floats, turn
the fuel bowl upside down, then adjust the fuel valve seat adjustment nut until the float sits 5/16”
from the bottom of the float bowl (the side with the adjustment nut), measured at the middle of the
float (a drill bit is handy to measure with). Tighten the slot-head fuel valve seat lock screw.
28. Install and tighten the slot head fuel level check plug, using the new check plug gasket from the kit.
29. Assemble the main body assembly, metering block and fuel bowl, using the new fuel bowl gasket
and metering block gasket from the kit. Whilst the fuel bowl gasket is normally symmetrical, the
metering block gasket is not, and care must be taken that it is not put in back-to-front – check the
alignment of the holes in the gasket with those in the metering block.
30. Install and tighten the four slot-head fuel bowl screws, using the new plastic bowl screw gaskets from
the kit.
31. Fit the pump operating lever, screw, spring, locknut and retainer.
32. Ideally, the accelerator pump operating lever should just be in contact with the short pump arm
(mounted on the accelerator pump cover) at idle. This absence of slack
gives sharp accelerator pump response. However, the accelerator pump
operating lever needs to be set such that it does not overflex the pump
diaphragm. To set the operating lever clearance, tighten the locknut such
that the pump operating lever is just in contact with the short pump arm.
Next, hold the throttle fully open, then move the pump arm until the pump
diaphragm is fully flexed. Measure the gap between the pump arm and the pump operating lever
adjustment screw with a set of feeler gauges. If the gap is less than 0.015”, back off the locknut to
suit. This will mean that there will be a slight throttle response delay (due to the operating lever
having to move a little bit before it contacts and starts to move the pump arm), but this is preferable
to overflexing (and tearing) the pump diaphragm.
33. Reassemble the three separate choke assemblies (fast idle pick-up lever, fast idle cam lever spring
and fast idle cam lever as one unit, back-up plate and stud assembly, choke rod lever and bushing
assembly and choke spring as a second unit and choke lever and swivel assembly, fast idle cam
plate and fast idle cam and shaft assembly as a third unit.
34. Install the fast idle pick-up lever, fast idle cam lever spring and fast idle cam lever as one unit and
hold it in place with the phillips-head fast idle cam lever screw.
35. Fit a new paper circle gasket from the kit to cover the choke vacuum passage. Holding the gasket in
place, fit the back-up plate and stud assembly, choke rod lever and bushing assembly and choke
spring as one unit. Install the choke link hairpin clip to the choke link.
36. Install the choke lever and swivel assembly, fast idle cam plate and fast idle cam and shaft assembly
as one unit. Install the single slot-head screw to hold it in place, then fit the choke cable holder
bracket with it’s two slot-head screws.
37. Adjust the fast idle screw with the choke fully open such that the gap between the throttle plates and
the throttle bores is 0.020”. This is pretty small to measure with a set of standard drill bits, so again a
“bent and narrowed” feeler gauge is handy.
38. Set the throttle stop screw such that the throttle plates are closed, then back the screw out 1½ turns.
39. Install the fuel filter spring, new sintered bronze filter and fuel filter gasket from the kit. Install and
tighten the fuel inlet fitting (1” AF), using the new gasket from the kit.
40. Before putting the carburetor onto the vehicle, it is wise to double check (triple check) that the parts
that can fall through into the carburettor throat are staked and/or tight:
Page 38 of 66
pump nozzle screw tight. Although the screw and nozzle cannot pass into the engine (won’t fit
past the booster venturi), the pump check valve “needle” certainly can.
hot air choke restrictor grub screw tight.
Choke plate screws tight and staked.
Throttle plate screws tight and staked.
41. Install the carburettor to the manifold using the new gasket from the kit. Do not tighten the flange
nuts just yet as the ability to move the carburettor slightly makes some of the connections easier.
Note that the throttle bores are not centered on 350 Holley carburettors, and are offset 0.17” (~11
/64”)
to the back of the flange. If the carburettor flange gasket is put in back-to-front, it may catch the
throttle plates, causing them to jam open
42. Connect the choke control cable to the choke actuation lever, and mount the outer sleeve to the
cable clamp. Actuate the choke cable through its full range of motion to ensure full choke operation
and adjust as necessary.
43. Connect the fuel line to the carburettor fitting.
44. Connect the manifold vacuum hose at the rear of the throttle body base, and the timed spark vacuum
hose at the side of the metering block. Note that in some cases either or both of these hose
connections may be not used, and must hence be capped off at the carburettor with plastic caps.
45. Install the throttle cable to the clamp and to the carburettor. Refit any throttle return spring(s)
required.
46. On automatic transmission vehicles only, install the transmission kickdown adjustment screw and
black retaining clip, as correctly indicated. Failure to attend to this detail may
result in a sticking wide-open throttle or dangerous uncontrolled engine
speed.
47. Tighten the manifold flange bolts to 15ftlb in the pattern shown in the image
to the right. Do not overtighten the nuts, as a warped or cracked throttle body
may result.
48. Check that the throttle operates smoothly and returns to idle. Check that wide
open throttle (WOT) is achieved.
49. Start the engine and check the fuel lines and inlet fitting for possible leaks.
50. Place the air cleaner gasket (not supplied in the kit) on the sealing flange, and install the air cleaner.
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9 Tuning
9.1 Fuel Level
Fuel level is adjusted so that the vehicle does not run out of fuel (lean out)
under cornering or acceleration (too low) or burp uncontrolled into the
engine (too high).
To set the fuel level:
1. Start the vehicle. 2. Remove the fuel bowl sight plug. 3. Observe the sight plug hole for the fuel level. If none is seen, the
level is too low - fuel should be even with the bottom of the sight plug hole. If fuel comes pouring out of the sight hole, the float is set too high.
4. To adjust the float level, shut down the engine. 5. Loosen the lock screw on top of the fuel bowl just enough to allow you to turn the adjusting nut. Hold
the screw in position with the screwdriver. 6. Turn the
5/8” AF adjusting nut in the appropriate direction: clockwise to lower float and
counterclockwise to raise float. Turn the nut in increments of ¼ of a rotation. 7. Retighten the lock screw. 8. Restart the vehicle and observe the sight plug hole. Repeat steps 4. – 7. as necessary. Note that there are a variety of Viton-tipped needle and seat combinations available for 350 Holley carburettors, ranging from 0.097” seat diameter to 0.110” diameter. Steel and titanium needle and seat assemblies (not recommended for street use but required for methanol use) are available up to 0.150” seat diameter. Installing too small a needle and seat means that the engine can starve under high load. Installing too large a needle and seat means that control over the fuel level will be more sloppy. The chart below shows the fuel flow through various needle and seat sizes at different fuel pressures. Note that for early Holdens (running at 3-6 psi of fuel pressure) there is no benefit in changing to a larger needle and seat size (even the 0.097” diameter seat will flow twice the early Holden fuel demand).
Page 40 of 66
0
10
20
30
40
50
0 1 2 3 4 5 6
Fue
l Flo
w (
GP
H)
Inlet Pressure (psi)
0.082"
0.097"
0.101"
0.110" (holes)
0.110" (windows)
0.120"
early Holden fuel demand
Page 41 of 66
9.2 Idle Speed and Idle Mixture
Engine idle speed (often referred to as “curb idle”, the speed the engine runs at when warm with the
choke off) is adjusted so that the vehicle does not stall when stationary (too low) or consume excess
fuel/jump when moving off (too high). Idle mixture is set to provide a good fuel/air combination (neither too
rich nor too lean) when stationary. Whilst idle speed and idle mixture can be set “by ear”, there are some
tools that make it easier/more consistent:
A tachometer (either dash mounted or fed from the ignition leads) can help accurately set idle speed.
If a tachometer is unavailable, a timing light can be connected and the number of “flashes” in twelve
seconds counted. Multiply the number of flashes by ten to get the RPM. This is pretty hard to do
though – you are looking to count around four flashes per second.
A vacuum gauge (either dash mounted or a removable pressure gauge that screws into the inlet
manifold after disconnecting the vacuum wipers (FB and earlier Holdens) or power brake/windscreen
washers (NASCO accessories) from the manifold. The vacuum gauge gives a more accurate setting
to the idle mixture than the “back it off until it runs smooth” method.
To set the idle speed and mixture:
1. Warm the car up to normal operating condition. Check the choke is off. Leave the air cleaner in
place.
2. Fit the vacuum gauge to a vacuum manifold port on the carburetor and the tachometer (where
available).
3. Adjust the curb idle speed screw until the engine idles at 480-520 rpm (check with a tachometer,
timing light counting or “by ear”).
4. Adjust the two idle mixture screws 1/8 of a turn at a time, alternating between each screw. Turn them
equally, until you achieve the highest possible vacuum reading without adjusting the curb idle speed
screw. If a vacuum gauge is not available, use a tachometer (or your ear) to obtain the highest
possible RPM.
5. Check the engine speed again, and repeat steps 3. and 4. above until a satisfactory idle is achieved.
6. Remove the tachometer and vacuum gauge and refit any vacuum lines that were disconnected.
If a rough idle persists after the mixture screws have been adjusted, check for vacuum leaks. These could
result from unplugged vacuum fittings, carburetor flange gaskets that were torn during installation,
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cracked lines or loose bolt/screws. A quick way to check vacuum leaks is to spray WD40 in the suspected
area with the engine running – if the engine speed increases, there is a vacuum leak.
Note that on vehicles with very lumpy cams, the large amount of valve overlap can mean that there is
very little vacuum at idle (6”Hg or less). At times, these vehicles may not respond well to setting the idle
mixture – the idle mixture screw seems to do little to help the idle. The low engine vacuum at idle means
that the throttle plates need to be opened more than usual to draw fuel from the idle system… in fact they
can be opened so much that the idle transfer slots are uncovered, leading to excessively rich idle (and no
control of the idle mixture as the idle needle valve only controls the lower idle discharge hole). To check
for this, set the idle as best as possible, then remove the carburettor and check the throttle plate position
– if more than 0.030” of the idle transfer slot is exposed, then this may be the cause of the loss of idle
control. One method to fix this is to drill a 1/16” hole in the throttle plate on the same side of the shaft as
the idle discharge holes. The small hole will allow some air to pass, allowing the throttle plates to be
closed further and idle mixture control regained. The hole can be enlarged by stepping up drillbits in 1/32”
increments until the throttle plates are sufficiently closed (don’t go too large or the throttle paltes will be
fully closed, giving an off-idle flat spot). This condition should not be confused with an early opening
power valve – see Section 8.8 below.
For some vehicles, even with the idle mixture screws turned all the way in (lean), it may not be possible to
obtain a satisfactorily lean idle mixture. In these cases the idle feed restriction may be closed up, or the
idle air bleed enlarged. Modifying the idle feed restriction is preferred, as it does not affect the timing of
the idle system. However, the 350 Holley idle feed restriction appears to be inside the idle tube, making
this a difficult task. Whilst it is simple to “drill out” the idle air bleed, this will cause the idle system to start
flowing later. Modifying the idle feed restriction and idle air bleed should not be taken lightly, and should
be avoided where possible.
Whilst early Holdens, being manufactured prior to July 1972, are generally not required to comply with
emission standards. However, from that date onwards, all petrol passenger vehicles (and derivatives)
were required, when new, to comply with a performance standard (ADR) that set limits for exhaust
emissions of hydrocarbons (HC), oxides of nitrogen (NOx) and carbon monoxide (CO):
ADR26 was introduced 1/1/1976, and captures the CO at idle test (limit of 4.5% maximum volume
CO).
ADR27, 27A, 27B and 27C applied to vehicles manufactured from July 1976 to January 1986.
Vehicles made in this period generally ran on leaded petrol and employed carburettors.
ADR37/00 covers the period from February 1986 to the present. Vehicles manufactured after
January 1986 generally run on unleaded petrol (catalytic convertors), with computerized engine
management systems, fuel injection.
A summary of the emissions requirements of each of the tests above can be found here:
http://www.infrastructure.gov.au/roads/environment/impact/emission.aspx. Most early Holdens will not
have to conform to the above. However, some engineers request the CO at idle test when vehicles have
been modified to the extent that they require an engineer’s report. It is important to note that the idle test
is normally done at idle (480-520rpm). There is an alternative “high idle” test, which is conducted at
2500rpm. This test, although usually not applied to early Holdens, will bring the main metering circuit into
play (i.e. tuning for the CO at idle test is made via the idle needle valve (59), tuning for the “high idle test”,
if it was ever applied, is by changing the main metering jet). To tune the idle circuit to meet a CO at idle
test, an engine exhaust analyser is used – these are discussed more fully in Section 2.5.4 below. When
tuning for emissions, a CO at idle reading of 1-3% should be targeted.
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9.3 Fast Idle Speed
Fast idle is the speed that the engine runs at when the choke is on. To set the fast idle speed, 1. Start the engine, allowing it to reach operating temperature. Manually
advance the throttle to just off idle. Push the fast idle cam up, so the fast idle screw is on the top step of the cam. This fast idle speed should be set to 1500-1600 RPM.
2. To adjust the fast idle speed, shut down the engine and hold the throttle in the wide-open position to expose the fast idle screw below the choke housing. Use a small ¼” open-end spanner for adjustment, turning the screw clockwise to increase the RPM or counterclockwise to decrease the RPM.
3. Start the engine, and recheck idle speed.
9.4 Accelerator Pump
There are three different tuning parameters that may be modified to tailor the accelerator pump action
(how the vehicle initially responds when you put your foot down):
a) Pump discharge nozzle size,
b) Pump cam profile (colour and position), and
c) Pump capacity.
The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump’s
capacity and the profile of the pump cam. The period of time that it will take for this pre-determined
amount of fuel to be delivered is affected by the pump nozzle size.
9.4.1 Pump Discharge Nozzle Size Holley accelerator pump discharge nozzles are stamped with a number which indicates the drilled pump
hole size. For example, a pump discharge nozzle stamped “35” is drilled 0.035". Pump nozzle sizes are
available from 0.025" to 0.052". Note that:
whenever a #40 (0.040)" or larger pump discharge nozzle is installed, the “hollow” pump nozzle screw
should also be used. This screw will allow more fuel to flow to the pump nozzle, assuring that the
pump nozzle itself will be the limiting restriction.
whenever a #37 (0.037") or larger pump discharge nozzle is installed, the 50cc accelerator pump
should also be used.
The following guidelines can be used to tune the pump discharge nozzle:
a vehicle that accelerates well at first then bogs down may be squirting all the fuel shot too quickly.
The fuel shot can have it’s dutation extended by changing to a smaller pump discharge nozzle.
a vehicle which initially hesitates then accelerates smoothly (or gives a lean backfire) may need more
fuel initially. This can be achieved by fitting a larger pump discharge nozzle.
when changing the pump discharge nozzles, jump three sizes at a time. For example if there is
currently an off-line hesitation with a #28 (0.028") pump discharge nozzle, try a #31 (0.031") pump
discharge nozzle.
Once a pump discharge nozzle size selection has been made the accelerator pump system can be
further tailored with the pump cam.
9.4.2 Pump Cam Profile and Position
Holley offers an assortment of different pump cams, each with uniquely different lift and duration profiles.
The cams are colour coded (see graph below), with the standard 350 Holley cam being Orange. The
cams are available as Holley part number 20-12. Switching cams will directly affect the movement of the
accelerator pump lever and subsequently, the amount of fuel available at the pump nozzle. The table
below gives the speed with which each cam delivers fuel, which has been drawn from the first chart
below. The second chart gives the fuel volume delivered by each cam.
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← F
aste
r
sh
ot
Pink
Black
Green, White, Red and Orange
Brown
Blue
Yellow
Whilst the table and charts gives some guidance, the best method of selecting the correct pump cam is
by trial and error – monitoring either “seat of the pants feel”, quarter mile times or circuit elapsed times.
Page 45 of 66
Page 46 of 66
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 2
Cam
in p
osi
tio
n 1
Cam
in p
osi
tio
n 2
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Pu
mp
Vo
lum
e (
cc)
Pump Cam and Position
Page 47 of 66
Note that there are two holes in each pump cam, numbered 1 and 2. Placing the screw in position #1
activates the accelerator pump a little early, allowing full use of the pump’s capacity. Generally, vehicles
which normally run at lower idle speeds (600-700rpm) find this position more useful because they can
have a good pump shot available coming right off this relatively low idle. Position #2 delays the pump
action, which is good for engines that idle at 1000rpm and above. Repositioning the cam in this way
makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting. In
some drag racing applications, the vehicle is staged at considerably higher rpm. Where the full
accelerator pump shot is required at the (high) staging rpm, the pump cam can be rotated such that the
cam is at the start of lift at the staging throttle position. The cam holes are then redrilled for this new
position.
Note that pump arm adjustment and clearance should be checked and verified each and every time the
pump cam and/or pump cam position is changed.
9.4.3 Pump Cam Profile and Position
The standard 350 Holley accelerator pump is a 30cc unit. The larger 50cc unit should be used:
where a #37 (0.037") or larger pump discharge nozzle is installed,
where the brown or yellow pump cams are used (using the 30cc pump with these pump cams can
cause the throttle to jam open).
The 50cc accelerator pump is available as a bolt-on conversion kit (Holley part number 20-11). The main
differences between the 30cc and 50cc pumps are as follows:
Item 50cc 30cc Image (30cc on right)
Accelerator pump cover
Stamped 34R2773
Stamped 34R2178
B
Pump diaphragm
135-14 – deeper
135-12 - shallower
Diaphragm return spring
4 coils 3 coils
Note that the pump diaphragm cover screws are
7/32” longer for the 50cc pump.
The accelerator pump cover is thicker in the 50cc unit. This may necessitate the use of a ¼” (or thicker)
spacer between the carburettor and manifold in order for the pump cover arm to clear the manifold.
Another method used is to carefully grind a crescent in the inlet manifold to give the pump cover arm a
slot to operate in.
9.5 Main Metering Jets
Holley main metering jets are broached, flowed, and stamped according to flow rate. The stamped numbers are reference numbers and do not indicate drill size (for example, #88, #89 and #90 jets all have a 0.104” diameter hole). As Holley jets are widely available, there is no need to redrill a Holley jet (in any case,
Page 48 of 66
modification of the chamfers – see the image above left - on the jet by drilling can make the resultant jet flow very different rates to what would be expected). Holley main metering jets are available in sizes from #40 – #156. A higher relative number indicates a larger jet size. Standard main jets have a 3% tolerance in flowrates for a given jet size and approximately 4½% flow difference between jet sizes. A range of close-limit jets are also available with 1½% flow difference between jet sizes. Changing to a larger or smaller jet will either richen or make leaner the carburetor’s fuel curve from part throttle to full throttle, respectively. When changing the carburetor jetting, it is recommended to jump two jet sizes (e.g. change from a #60 to a #62 main metering jet). As there is only 4½% flow difference from one jet size to the next, changing one size won’t make that much of a difference.
Listed below is a recommended jetting range for various Holden engine sizes using the 350 Holley carburetor. This jetting size is a good general guide only and will vary from engine to engine depending on the degree of modification.
Engine Capacity
(Ci)
Main jets
Without venturi sleeves With venturi sleeves
149 #55 #49
161 #57 #50
179 #58 #51
186 #60 #53
192 #60 #53
202 #61 #54
If a vehicle is changing operation to a higher altitude, air is thinner so decrease the jet size one number for every 2000’ (600m) increase in altitude.
There are a number of ways to select the correct main metering jet (or correctly adjust an adjustable main metering jet):
reading the spark-plugs,
measuring exhaust gas carbon monoxide, and
running the car on a dyno/strip (more applicable to the power bypass jet – see below).
Each of these methods should be undertaken in conjunction with road testing, looking for stumbles, flat spots, drivability and fuel consumption.
Reading the colour of the spark plug electrodes (and to a lesser extent the colour of the exhaust pipe) provides a cheap and easy guide to correct main metering jet choice. This technique involves driving the vehicle for a run (up to operating temperature and a moderate distance at “cruise” conditions – not all at idle or full throttle!). After stopping then cooling down the engine, each plug is removed in turn and the colour of its electrode compared. Today the use of unleaded fuels and high-energy ignition systems has made this method much harder because very little color is seen on the spark plug; however the pictures below give some guidance:
Page 49 of 66
A more full description of spark plug readings can be found at http://www.classiccarhub.co.uk/articles/spark_plugs.html. A much more accurate way to tune the main metering jets is to measure the carbon monoxide (CO) in the
vehicle exhaust. CO is one of the gases in the engine exhaust (along with nitrogen (N2), carbon dioxide
(CO2), water (H2O), hydrocarbons (unburnt fuel, often written as HC), and various nitrogen oxides (NOX)
and sulphur oxides (SOX). The amount of CO in a vehicle exhaust is an indicator of the air/fuel mixture
being supplied to the engine, and thus is an excellent way of tuning jet sizes on carburettors.
Manufacturers typically specify a CO level somewhere within the range 0.5% to 3.5% by volume. At CO
levels higher than this there is a loss in economy, and at very rich settings, typically 8% to 10% CO, the
onset of poor running occurs, characterized by the particular engine sound that is known as “hunting”. It
should be noted that an engine, even in good overall condition, will show a fluctuation in idle CO over a
period of time, of typically 0.5%. To measure CO, a sample probe is placed into the exhaust pipe and an
exhaust gas analyser unit “reads” the CO in the exhaust. The other readings that some exhaust analyzers
provide include HC (the best mixture gives you the lowest HC), CO2 (the best mixture gives you the
highest CO2 reading) and O2. Whilst workshop units can cost in excess of $4000, a simple and cost
effective exhaust analyser (the “Gastester Digital”) is available from Gunsen for around $250 (see
http://www.gunson.co.uk/item.aspx?item=1835). This would not be a bad investment if you are
planning to tune a few early Holdens over the years. Using this analyser, some starting points for tuning
would be to tune to 0.75-1.25% CO (1–3% CO for a lumpy-cammed engine) at cruise conditions.
9.6 Power Valves
There are two different parameters that may be set for the power enrichment system – the time (or vacuum) at which it comes on, and the amount of fuel which is flowed. The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational. In this case, all manifold vacuums below 6.5” Hg, the power valve is operating. The factory “out of the box” power valve for 350 Holley carburettors is a standard-flow 85 (Holley part number 125-85). For most early Holden applications, a 65 power valve is suitable, provided
Overly lean (main metering jet is too small). Whitish or pale deposits. May also be seen by erosion of the spark plug electrode or detonation damage of the insulator.
Correct jetting: electrode deposits are slight and not heavy enough to cause any detrimental effect. Colour is brown to greyish tan colour, and minimal amount of electrode erosion.
Overly rich (main metering
jet is too large): Soft, black,
sooty deposit.
Page 50 of 66
the manifold vacuum is 12”Hg or higher. However, vehicles with a large overlap cam can idle at 6.0”Hg. At this vacuum, the power valve has opened and is starting to feed the mixture, leading to the vehicle “loading up” at idle. To correct this problem, install a lower-numbered power valve (e.g. a 55, 45 or 35 power valve). If the engine has a manifold vacuum of 12”Hg or less, a simple way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer is the power valve size. For example a vehicle with an idle manifold vacuum of 9”Hg, a power valve of (9 / 2 =) 4.5 is reasonable. Holley power valves come in two different types – a standard flow and a high-flow. The high-flow power valves will flow more fuel, though it should be noted that the power valve does not usually control the fuel flow – the Power Valve Channel Restrictions (PVCRs) do.
The following table gives the Holley power valves available:
Part No. Flow Vacuum opening (“Hg)
125-1005 High 10.5
125-105 Standard
125-95 Standard 9.5
125-185 High 8.5
125-85 Standard
125-75 Standard 7.5
125-165 High 6.5
125-65 Standard
125-155 High 5.5
125-55 Standard
125-50 Standard 5.0
125-145 High 4.5
125-45 Standard
125-135 High 3.5
125-35 Standard
125-125 High 2.5
125-25 Standard
125-10 Standard 1.0
As indicated above, the amount of fuel which is flowed is determined by the PVCR. These are located in
the metering block, and are able to be drilled to larger sizes to richen the fuel mixture under load. PVCRs
which have been over-enthusiastically drilled and are too large can be reduced in size by inserting small
vee-shaped lengths of wire into the PVCR.
One method to tune the power valve (PVCR diameter and best vacuum opening point) is to use timed acceleration runs (e.g. ¼-mile times), or top speed/power (e.g. dyno-tuning). This involves trial and error jetting changes to obtain the best results, and needs some moderate track or dynamometer time to get decent repeatable results. An easier way is to again tune using an exhaust analyser (particularly if you have the Gunson exhaust analyser described in above). Some starting points for tuning would be to tune to 6.6% CO under load conditions. Whilst this could be reduced to 4% for engines with very good combustion chamber design, early Holden cylinder heads rarely meet this criteria. For vehicles that run wide-open throttle most of the time (like HQ Holden circuit racers), the power valve is often removed (blanked off with blanking plug part number 26-36) and the main jet sizes increased 6-8 jet sizes to suit. Whilst this is suitable for full throttle performance, it will lead to a very rich “cruise” condition and is not recommended for street use.
Page 51 of 66
If a vehicle is changing operation to a higher altitude, less vacuum is made by the engine so decrease the power valve setting 1.5-2”Hg for every 3000’ (900m) increase in altitude. Note that it is possible to damage the power valve by engine backfire. For carburettor built after 1992, a
power valve blow-out protection system (a ball check valve is located in the throttle body, designed to be
normally open but which quickly seats to close off the internal vacuum passage when a backfire occurs)
is installed. Once closed, the check valve interrupts the pressure wave caused by the backfire, thus
protecting the power valve. A kit is available to retrofit the power valve blow-out protection system to pre-
1992 350 Holleys – see below.
9.7 Venturi Sleeves
Venturi sleeves increase air speed through the venturi, which help prevent flat spots and assists with low-down acceleration. Venturi sleeves should be used on Holden 149 and 161 engines when using a Holley 350 carburetor. Main jet sizes must be reduced when using these sleeves (see table above). It is also recommended to close up the two power valve channel restrictions in the metering block from 0.060” to 0.030”. This can be achieved by inserting bent vee-shaped wires or by fitting brass bleeds, though this can be a difficult process and it may be worthwhile putting up with a slightly rich power system for everyday (not race dedicated) use. Redline Performance venturi sleeves are available from American Auto Parts (part number 14-35) and Barnes Performance (part number BP14-35). The Redline Performance venturi restrictors are 0.035” thick (~
1/32”), and will change a Holley 7448 venturi diameter from 1
3/16” diameter to 1.118” diameter (~1
5/32”).
The venturi restrictors are fitted into the top of the venturi (with the gaps over the booster venturis) and epoxied in place. The gap in the sleeve does not close up – it leaves a gap down the venturi wall. Note that venturi sleeves will slightly reduce the flow capacity (the 350 Holley is reduced from 350CFM to approximately 320CFM). To install venturi sleeves:
1. Whilst it’s possible to install the venturi restrictors with the carburettor still on the vehicle, there is a fair
risk that you will drop something down the carburettor throat. It is strongly recommended that the
carburettor be removed before fitting the venturi restrictors.
2. Clean the venturis and the outside of the venturi sleeves with some thinners to remove any coke,
grease or fuel.
3. Unscrew the retaining screw and remove the accelerator pump discharge nozzle, screw and gaskets.
4. Place a small piece of PVC tape across the top of the accelerator pump discharge hole and another
one across the four air bleeds. This will stop any stray epoxy falling into places it shouldn’t.
5. Drop the first venturi sleeve down past the choke plate. It’s a lot
easier if the choke plate is removed, but not impossible (and
restaking the choke plate screws is a pain in the bum). Resist the
temptation to put the epoxy onto the venturi sleeve before dropping
it in – there is a fair amount of twisting and wiggling to get the
restrictors past the choke plate, which would get epoxy everywhere.
6. Turn the venturi restrictor around so that the slot lines up with the
booster venturi, then push the restrictor into place (dummy fit). It
should be a nice snug fit. The lip on the top of the restrictor stops it
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from pushing down too far down into the venturi (…or into the motor!).
7. Turn the carburettor over and use
a screwdriver to gently poke the
restrictor back out of the venturi,
leaving it sitting on top of the
booster/venturi.
8. Mix some epoxy, and using a cotton bud (or similar spreader) paint the outside of the venturi sleeve
with epoxy. Take care not to get epoxy near the carburettor walls. Use only a thin smear of epoxy and
don’t spread it too close to the edges of the venturi restrictor as it will smoosh out later. Avoid the
temptation to fit and epoxy both restrictors at once… it is a fiddly process, easy to bugger up and the
epoxy goes off quickly.
9. Push the freshly epoxied venturi restrictor into place. Check for any smooshed epoxy (or smears on
the carburettor walls) and wipe it out while it is still wet.
10. Repeat steps 4 – 8 for the second venturi restrictor.
11. Allow the epoxy to set thoroughly. Remove the PVC tape and reinstall the accelerator pump discharge
nozzle.
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10 Troubleshooting
It is common knowledge that “carburettor” is French for “don’t F%@# with
it”. Many Australian children have learnt to swear from listening to the
carefully phrased epithets gently wafting from the open bonnet of an early
Holden. The guidance below may assist in hunting down the cause of
early Holden Stromberg issues (and perhaps prevent your children from
developing their vocabulary). Of note, many ignition and timing issues are
found to be the real cause of what is perceived to be a “bad carby” – the
following table assumes all electrical and timing issues have been
resolved.
Cause
Problem
Sta
lling
Roug
h idle
Flo
od
ing
Hot S
tart
Econo
my
Hesitatio
n
Accele
ratio
n
Surg
e
Back fire (
co
ld)
Pow
er
Sta
lling (
co
ld)
Incorrect idle adjustment
Damaged idle needle
Incorrect fast idle adjustment
Idle passages blocked
Metering jets loose or blocked
Power valve loose or sticking
Fuel inlet needle and seat loose or passing
Float leaking, rubbing or level incorrectly set
Gaskets leaking
Pump discharge holes blocked
Pump diaphragm worn or cut
Pump check ball dirty or sticking
Choke valve and linkage dirty, sticky or damaged
Throttle valve loose, damaged or sticking
Venturi dirty
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11 Modification
11.1 Fuel Supply Stability
11.1.1 Wedged Float
For vehicles which are frequently turning under load (for example
Speedway use and circuit racing), fuel can slosh to one side of the float
bowl. The fuel acting on one end of the float causes it to rises, prematurely
cutting off the fuel supply. To prevent this, a wedged float is available (part
number 116-13). The wedged float is made from nitrophyl, and as the
name suggests has a wedged shape. The wedge is designed so that as
fuel sloshes up the side of the bowl, the fuel will ride up the wedge and
allow the float to stay open and not close off prematurely. The wedged
float is a bolt-in replacement for the original 350 Holley float.
11.1.2 Float Bowl Vent Baffle (Whistle)
For vehicles which are under frequent acceleration (for example drag
racing use), fuel can slosh to the rear of the float bowl and burp out the
float bowl vent. This can lead to overly rich mixtures as the burped fuel is
not metered. To prevent this, a float bowl vent baffle (often called a “vent
whistle”) can be added to the metering block. The vent whistle (part
number 26-89) extends the vent to the front of the float bowl, which is
normally “dry” under hard acceleration. To fit the vent whistle,
1. Remove and dissassemble the metering block.
2. Insert the vent whistle into the metering block
vent hole, protruding towards the float bowl
side.
3. Drill through the top of the metering block
and the top of the vent whistle with a #51 drill
(0.067”).
4. Insert the supplied drive screw through the
metering block and vent whistle. Ensure that
the top surface of the vent whistle is not
depressed (bent down) after installing the
drive screw.
5. Trim the end of the vent whistle so that it fits into the float bowl and has clearance to the front edge
of the bowl.
6. Check that the vent baffle has not sagged as it may contact the float, causing flooding. If it has
sagged, stake the metering body underneath the baffle to raise it.
7. Blow out the metering block with compressed air to remove any drill swarf before reassembly.
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11.2 Higher Air Flow
11.2.1 Choke Horn Removal
One technique for squeezing more airflow (around 30-50 CFM) from a 350 Holley carburettor is to remove the choke air horn and blend the radius of the resulting edge. Whilst the process is simple, it does have it’s downfalls:
Pro Con
More airflow is gained (30-50CFM). The flow increase is small, and may be made more reliably by changing to a 500CFM Holley.
Bonnet clearance is increased. The process is not reversible, unlike the use of a K&N Stubstack (see below).
Material cost is low. The flow increase may be detectable on a dynometer, but is not likely to be noticed “by the
seat of the pants”.
Retains the 350 Holley carburettor (important where carburettor choice is restricted, such as in
Australian Speedway Production Sedan and Modified Production Sedan classes)
The change does not allow the use of the choke, making starting in colder locations difficult in
winter.
To get the full benefit, some epoxy needs to be added to the top of the carburettor air-horn. This
may be susceptible to cracking off (and falling into the running carburettor throat) and hence required
periodic checking.
The process is undertaken as follows: a) Disassemble the carburettor such that the main body assembly is bare. b) Scribe or stamp the List number and date code onto the underside of the air filter mounting plate (the
next guy who tries to work out what the carburettor originally was will thank you for it). c) Hold the main body in a vice (using wooden jaws/packing to avoid marking the faces) and cut off the
choke horn a few millimeters above the top of the main body with a hacksaw. d) Blend the top edges into the venturi with a die-grinder. e) Fill any resultant rough edges (including the now-redundant choke rod hole) with epoxy and sand
smooth. f) Wash out any filings in petrol, then blow through all orifices and channels with compressed air.
Note that in the photographs above I have not shortened bowl vent – this can be brought down to approximately
7/8” tall and mitred similar to the original.
It is important to realize that the increased airflow gained from removing the choke horn can change air/fuel ratios – the carburettor should be retuned after installing one.
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11.2.2 K&N Stubstack
An alternative to removing the choke horn in search of more airflow is to use the K&N Stubstack (K&N part number 85-021 for 350 Holley carburettors). The Stubstack is designed to increase the airflow of carburetors (typically 20-40 CFM, Hot Rod magazine's testing showed an increase of 28 CFM on a 650 Holley and 40 CFM on an 850 Holley, other testing has found 30 CFM on an 850 Holley) by decreasing the restriction around the choke horn. It reduces turbulence, improving metering accuracy. The Stubstack fits inside the air cleaner housing and slides snugly down over the choke horn. It is installed by loctiting into place on the choke horn whilst holding the air filter retaining nut finger tight. When installed on some models, there may be a slight gap at the base, however this is normal and will not affect performance. Due to space limitations between the choke horn and some air cleaner baseplates, the Stubstack has two thin spots in the casting. These will sometimes crack or chip slightly, but does not affect the performance. Whilst the Stubstack has been designed to work effectively with K&N Filtercharger™ elements and 360° custom air cleaner assemblies, many enthusiasts have found that it works equally well with other filters provided the minimum height above the stack is at least 1½" (preferably 2-3”), with best results obtained with large diameter filters of 4-5” height. This is not always possible with low bonnet clearances. It is important to realize that the increased airflow gained from installing a Stubstack can change air/fuel ratios – the carburettor should be retuned after installing one.
11.3 Automatic Choke
350 Holley carburettors came from the factory with manual chokes. The manual chokes fit well with the
FB/EK Holden dash, as the original Holden choke knob and cable can be used to drive the 350 Holley
choke and looks factory. However, for ease of driving it is possible to convert the choke to either electric
or hot air. Whilst conversion to electric choke is the far simplest option, I will cover both electric and hot air
chokes here for the sake of completeness.
11.3.1 Automatic Choke Operation
Electric and hot air chokes operate almost identically – the only difference is the heat source they use.
Just like the manual choke, the choke plate is connected by a link rod that passes down through the air
cleaner mounting boss to the choke assembly. The rod is connected a choke housing lever. The lever is
put under tension by a coil spring which holds the choke in the closed position when the coil spring is
cold. If the coil is warmed up, it expands and slowly moves the choke housing
lever/choke link rod/choke plate to the open position. The amount of tension in the
spring determines how much heat is required to get the spring to start moving. The
tension can be adjusted (by manually winding the spring up or down) by rotating
the choke housing cap a few degrees either way. A series of marks on the cap
show just how far it has been turned. One mark (the “index mark”) is normally
bigger than the others and is used as the reference point. Rotating the cap
anticlockwise to make the choke stay on longer is referred to as “richer”, whilst rotating the cap clockwise
to make the choke open earlier is referred to as “leaner”. Both RICH and LEAN are cast into the choke
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cap as a reminder. The choke coil must be adjusted so that it opens the choke plate at the proper speed
as the engine warms up - slowly enough to prevent lean stumbles; yet fast enough to prevent over-rich
mixtures.
The heat source for a hot air choke is the engine warmth. A small amount of
air is drawn from a clean place, often by tapping into the air cleaner (the
image to the right shows a Model 2300 carburettor where the air cleaner
bosses have been drilled and a tube fitted to allow clean air to be drawn from
the air cleaner). The clean air is then routed past a source of heat – often a
choke stove in the exhaust manifold or inlet manifold crossover. If the engine
is cold, the air is also cold, As the engine warms up, so too does the air flow.
The “hot air” then passes into the choke housing past a small brass piston (we’ll come back to that small
piston later) and past the coil spring. The “hot air” provides the heat to warm up the coil spring. The spent
air then flows through a small vacuum port in the side of the carburettor (see red arrow in diagram to the
right above). This port is present in all 350 Holley carburettors,
though with manual chokes fitted the port is either covered with a
gasket or is lead-filled. The air flows down a channel in the main
body, through a channel in the throttle body and into the inlet
manifold (manifold vacuum is used to “suck” the air along). A
small screw-in brass restrictor (0.055” diameter or #54 drill) in
the throttle body is used to control the rate of the air flow (the
orifice is present in all 350 Holleys, including those with manual
chokes – see red arrow in the diagram to the right). For an
electric choke, the same air flow occurs, but the air source is not
heated (it is often still drawn from the air cleaner, but does not
pass through a hot air stove). Here the air flow is used to prevent the electric choke coil from burning out.
Heat for the electric choke is supplied by electricity warming the coil (just like a toaster element). The
electrical power is turned on when the vehicle ignition is switched on, starting to slowly warm and unwind
the choke spring. Even when the engine is hot, the choke spring normally has power supplied to it.
Without the air flow, the choke coil would soon burn out.
The air flow path can be seen in the image to the
right:
1. hot air enters the choke housing,
2. passes through a channel and into the main
housing compartment,
3. flows over the choke coil (removed in this
photograph) which warms the coil,
4. flows past the choke pull-off piston,
5. passes through a further channel, and
6. exits the choke housing to flow to the throttle
body.
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Having the choke plate fully shut is fine for initial starting as it puts a lot of vacuum on the fuel system (at
the low engine speeds developed by the starter motor there is precious little vacuum, so closing the
choke plate helps conserve what little there is). However, when the engine first fires the vaccum available
is much larger, and the engine requires to be fed some air. A fully closed choke plate will cause the
engine to be overly rich, and can stall or flood. To prevent this, the choke plate is cracked open slightly as
soon as the engine fires. This is done by the small brass piston mentioned above, called the choke pull-
off. The choke pull-off has the air flowing past it for the choke supply. At low engine speeds (when the
starter motor is turning) there is not enough vacuum (and hence not enough air flow to the choke) to
move the small brass piston. Once the engine fires, vacuum increases (as does air flow to the choke).
This causes the choke pull-off piston to move, overriding the choke coil spring and cracking open the
choke plate. After a minute or so of operation, the choke coil spring has
warmed up and opens the choke plate even more, making the choke
pull-off redundant. Note that the choke pull-off piston only just cracks
the choke plate open – if it opens too much, the engine could hesitate,
backfire through the carburettor or stall from having too little choke
function. The amount the choke plate is cracked open by the choke
pull-off is adjustable, often by bending a linkage rod or by an
adjustment screw. The choke pull-off can be seen in the image to the
right.
The choke linkage also incorporates a fast idle cam (the red plastic
item in the image to the right). The fast idle cam bumps open the
throttle a small amount when the choke is opened, increasing engine
speed. The fast idle cam has a number of “steps” that are ridden by the
fast idle screw. As the engine warms and the choke closes, the fast idle
cam rotates, the fast idle screw drops down to lower “steps” and the
throttle closes back to the curb idle speed. When the choke pull-off
cracks the choke plate open, pushing the accelerator pedal allows the
cam to rotate so that the fast idle screw will drop from the highest
(fastest) step and align with the second highest (second fastest) step of
the fast idle cam. Note that adjusting the choke pull-off may also
change the fast idle cam position and vice-versa.
If the vehicle has flooded, the spark plugs will be wet and will prevent the engine firing. If the accelerator
pedal is pushed all the way to the floor (and held there), the throttle will rotate open ready to let lots of
lean fuel/air mixture in to dry the plugs. However, the closed choke plate will prevent this air getting in. To
accommodate this, the fast idle cam lever has a small unloader tang on one side. By pushing the
accelerator pedal all the way to the floor, the rotating throttle shaft rotates the fast idle cam lever, the tang
moves forward and butts up against the end of the “steps”, pushing the fast idle cam partly around. This
drives the choke plate manually open (not all the way, but a little bit more than the choke pull-off would
open it). This allows the airflow in to clear the flooded engine. Releasing the throttle allows the choke
spring (or choke pull-off) to resume controlling the choke plate position.
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11.3.2 Electric Choke Conversion
An electric choke kit is made by Holley (part number 45-
223) to replace the manual choke. The kit supplies the
following parts:
a) short black earth lead,
b) long red (sometimes black) 12V positive lead,
c) thermostat housing gasket
d) thermostat housing clamp
e) thermostat housing assembly
f) choke housing assembly
g) choke link
h) fast idle cam lever
i) fast idle cam lever spring
j) choke housing gasket
k) choke link retainer
l) choke housing screws
m) fast idle cam lever screw
n) thermostat housing clamp screws
To install the electric choke kit: a) Remove the carburetor from the vehicle. b) Remove the three choke housing screws securing the manual choke
housing assembly to the main body. Remove the choke link retainer from the choke link and the manual choke backing plate. Keep the retainer for use at a later time.
c) Remove the fast idle cam lever screw. Remove the fast idle cam lever
and fast idle pickup lever and fast idle cam lever spring. Retain the screw, spring, and small pickup lever.
d) Pull the choke housing gasket off and discard it. If the choke vacuum passage has been lead-sealed, carefully drill through the lead (with a drill in a hand chuck) then pick out the hollow ball. Thoroughly clean the mounting surface of the gasket. Blow compressed air through the passage from the bottom of the throttle plate up to make sure the passage is free of blockages. Air should exit from the choke passage.
e) Use the new fast idle cam lever from the kit and the screw, spring, and small pickup lever retained above, as
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shown in the figures below. Assemble the parts onto the throttle shaft. f) Moisten the cork choke housing gasket provided in the kit. Attach it to the vacuum passage hole on
the choke housing assembly. Insert the choke rod through the hole in the lever on the back of the choke housing. Make sure the fast idle cam is above the choke rod. Use the fastener clip retained above to secure the rod to the lever. Position the choke housing assembly to the carburetor main body. It will help to open the throttle slightly to clear the fast idle lever away from the fast idle cam assembly. Note that on most applications, the original choke link will be used. However, on certain carburetors the new choke link supplied with the kit may have to be used.
g) Using the three equal-length long choke housing screws from the kit, secure the choke housing assembly to the main body. Manually operate the choke plate by moving the bi-metal pick-up lever on the front of the choke housing assembly. The choke plate should move freely. If not, check the choke linkage to make sure there is no binding and that the fast idle screw is in alignment with the cam on the back of the choke housing.
h) Install the new thermostat housing gasket onto the thermostat housing.
i) Install the thermostat housing and thermostat housing clamp. Install the clamp so it bows outward from the housing as per the image to the right. Ensure the bi-metal pick-up lever (in the housing) fits into the loop on the bi-metal spring. Check this by turning the housing in both directions. The choke plate should open when rotated clockwise, and it should close when rotated counter-clockwise.
j) Using the three equal-length short thermostat housing clamp screws from the kit, fasten the clamp and thermostat housing to the choke housing assembly. Tighten it enough to hold the thermostat housing in place, but still allow it to be rotated.
k) Rotate the thermostat housing until the mark on the housing aligns with the index on the choke housing assembly. Tighten the clamp screws so the housing cannot rotate. Do not block the fresh air intake.
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l) Correct polarity must be observed when connecting the electric choke wires. Connecting the (+) lead to earth and the (-) lead to a 12V source will result in a direct short and could cause a fire. Use the shortest wire from the kit to connect the bayonet end to the thermostat housing negative terminal marked (-). Ground the eyelet end to the carburetor. Do this under a screw securing the choke housing assembly to the main body.
m) Remount the carburetor on the vehicle. Connect the long wire from the
kit to the thermostat housing positive terminal marked (+). Run the other end of the wire through the firewall using an existing grommet. Connect is to an ignition activated 12V source so that the choke cap only gets voltage when the engine is running – for FB/EK Holdens, this is the from the fuse panel (located under the dash on the driver’s side) via the 15 amp turn signal/heater/backup fuse (the lower one on the picture to the right) at the back of the panel. A blown choke fuse will disable the heater fan, indicators and reverse lights (each of which were either options or standard on FB and EK Holdens). The choke power lead will require a female spade connector on the end to connect onto the male terminal blade at the rear of the fuse panel (a simple push on fit as the male terminal is already present in all FB/EK Holden fuse panels). It is recommended that insulated terminals are used, as many of the FB/EK wiring terminals are bare, and easy to short. Double check with a voltmeter or test light that the choke only has power when the ignition is on. Note that it is not recommended to use the 12V side of the coil for the power source, as it will result in unacceptable choke operation, and could cause engine misfiring, resulting in possible engine damage. If the vehicle has an aftermarket oil pressure switch (for example driving an electric fuel pump), it would be even better to route the power through that switch – that way the choke only gets power once the engine has fired (and has oil pressure), and does not get power (and start opening) if the vehicle is hard to start and cranking for a long time.
n) Start the engine, allowing it to reach operating temperature. Manually advance the throttle to just off idle. Push the fast idle cam up, so the fast idle screw is on the top step of the cam. This fast idle speed should be set to 1500-1600 RPM. Shut down the engine, and hold the throttle in the wide-open position to expose the fast idle screw below the choke housing. Use a small ¼” open-end spanner for adjustment, turning the screw clockwise to increase the RPM or counterclockwise to decrease the RPM. Start the engine, and recheck idle speed.
o) Choke tuning adjustments are listed below. After making final adjustments, start the engine and make sure the choke plate opens completely.
11.3.3 Hot Air Choke Conversion
Whilst a genuine kit is not available, it is possible to fit the 350 Holley carburettor with a hot-air choke from
another Holley carburettor. This tends not to be a common modification though, as it is easier to source a
12V power source for an electric choke than it is to tap a hot air source. The hot air choke operates
similarly to the electric choke described above, with a coiled spring being connected to the choke rod.
When the engine is cold, the spring holds the choke plate closed. A flow of air is drawn into the choke
housing, past the coil and into the carburettor. The air is drawn from a source that will heat up as the
engine does – often by taking air from the air filter and passing it over the exhaust manifold before
feeding to the choke housing. As the vehicle warms up, the hotter air heats the coiled spring. The spring
expand, moving the choke rod to open the choke plate. It can be a challenge to find a source of hot air in
aftermarket manifolds. One solution is to draw air from a clean source (preferably inside the air filter) via a
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thin ductile copper tube. The copper tube is wrapped a few times around the exhaust manifold (or an
extractor pipe) before being connected to the choke housing. The choke housing thread is a weird one,
though a 12.7x0.91 annealed copper tube (from Bunnings) fits over it with a little persuasion..
11.3.4 Automatic Choke Tuning
The following tuning points are available for Holley automatic chokes:
a) Adjusting how much the choke plate pulls open when the engine first fires.
As a starting point, the factory tuning settings can be used. The integral chokes which are bolted
onto 350 Holley carburettors are normally of two types – either with an adjusting screw, or without
one.
If the adjustment screw is present on the side of the choke housing, the following can be used as a
starting point:
Dig the caulking out of the adjustment screw with a
sharp pick.
Remove the choke cap and push the choke pull-off
piston inwards (onto the adjustment screw
shoulder).
Apply light closing pressure to the choke plate,
then use drill bits to measure the gap between the
top edge of the choke plate and the air horn wall
(put the drill bit in parallel to and adjacent to the air
horn vent, but up against the carburettor wall).
Adjust the screw in or out to give a gap of
approximately ¼”. Turning the adjuster screw
counter-clockwise (out) will open the gap, turning
the adjuster screw clockwise (in) will close the gap.
Note that the adjustment screw should be sealed over
again once tuning is finished to prevent vacuum leaks.
If the adjustment screw is not present on the side of the choke housing, the following can be used as
a starting point:
Bend a paper clip so that it has
an 1/8” end.
Remove the choke cap and
insert the paper clip into the
end of the choke pull-off piston.
Feeling gently, hook the paper
clip into the piston bore slot.
Move the piston in until the
edge of the piston slot engages
the paper clip. The piston is
now “pinned” into the bore by
the paper clip.
Apply light closing pressure to
the choke plate, then use drill bits to measure the gap between the top edge of the choke plate
and the air horn wall (put the drill bit in parallel to and adjacent to the air horn vent, but up against
the carburettor wall).
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Bend the piston lever tang to give a gap of approximately ¼”.
Once this basic setting has been made (and the choke cap reassembled), start the vehicle and
observe how the vehicle performs for the first thirty to sixty seconds:
If the choke plate refuses to move at all when the engine first fires, there may be a number of
reasons:
o the cork seal is installed in between the main body and housing may be missing,
o the hot air supply may be blocked with carbon, or a lead ball,
o the linkages or the piston could be binding, or
o the vehicle may have a very, very lumpy cam and make insufficient vacuum to move
the pull-off piston.
If the engine is running overly rich (black smoke, strong smell of unburnt fuel, or rich-stalls), adjust
the choke pull-off to open the choke plate a tiny bit more.
If the engine hesitates, backfires or lean-stalls, adjust the choke pull-off to close the choke blade
a tiny bit more
Be careful, as a tiny change in the choke pull-off is amplified by the linkage and makes a big
difference to the choke plate position.
b) Adjusting how long it takes the choke to start opening once the
engine has fired.
The choke plate should be tightly shut when the engine is cold.
With either of the hot-air or electric chokes, start the choke coil
adjustment by turning the mark on the choke coil to the index
position on the housing. To adjust the choke coil, start the vehicle
and let it run for thirty to sixty seconds.
If the engine hesitates, backfires or stalls after the thirty to sixty
seconds, the choke is probably opening too soon. Loosen the
three lock screws (see red arrows in the image above) and turn the choke cap one index mark
anticlockwise (RICHER). Tighten the three lock screws, let the engine cool all the way down then
repeat the tuning.
If the engine is running very rich (black smoke or the smell of unburnt fuel) after the thirty to sixty
seconds, the choke is probably opening too late. Loosen the three lock screws and turn the choke
cap one index mark clockwise (LEANER). Tighten the three lock screws, let the engine cool all
the way down then repeat the tuning.
If the choke plate won’t open all the way even long after the engine has warmed up, the problem
issue is almost certainly a lack of heat to the choke coil - blocked exhaust crossovers, missing or
defective heat riser valves, blown electrical fuses or blown electric coil.
c) Adjusting the unloader tang.
When the engine is cold and the choke is closed, check that the
fast idle cam is on the highest step. Push the accelerator slowly to
the floor. The choke plate should be observed to be mechanically
forced open at least as much as the choke pull-off would open it
(about 11
/32”). Although not critical, the amount that the choke is
forced open can be adjusted by bending the unloader tang. The
tang is pretty heavy, and buried under all the fast-idle linkages – it
is easier to remove the choke housing first before bending the
unloader tang. The image to the right shows the fast idle screw
(item 1) and the unloading tang (item 2). Bending the unloader tang in the direction of the green
arrow will crack open the choke plate more.
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11.4 Power Valve Blowout Preventer (Check Ball).
It is possible to damage the power valve diaphragm by
engine backfire. For carburettor built after 1992, a power
valve blow-out protection system (a ball check valve is
located in the throttle body, designed to be normally open
but which quickly seats to close off the internal vacuum
passage when a backfire occurs) is installed. Once
closed, the check valve interrupts the pressure wave
caused by the backfire, thus protecting the power valve
diaphragm. A kit (part number 125-500) is available to
retrofit the power valve blow-out protection system to pre-
1992 350 Holleys. The kit contains a drill bit with stop
collar, two check balls, springs and retainers and is installed with the carburettor removed from the
vehicle and the throttle plate removed from the main body. Note that two balls/stops/retainers are
supplied as the kit is also used on four-barrel carburettors with secondary power valves. To install the kit:
1. Clamp the throttle body gently in a drill press, using soft faces to protect the aluminum.
2. Check that the stop collar is mounted 0.300” from the supplied drill tip and is tight.
3. Locate the power valve passage on the top face of the throttle body (circled in red in the image to the right). Drill the passage larger, until the stop collar contacts the throttle body.
4. Remove the throttle body from the drill press and blow out all passages with compressed air to remove any drill chips.
5. Install the spring (tapered side facing up) into the newly drilled power valve passage, followed by the check ball.
6. Tap the spring retainer into place, flush with the surface of the throttle body.
11.5 Better Fuel Metering (Adjustable Metering Block)
An adjustable metering block is available for 350 Holley carburettors (Holley part number 134-276),
identified by the numbers "12323" stamped into the casting on its throttle lever side. The metering block is
supplied with all Keith Dorton Signature Series 350CFM carburettors, and is used to tune the fuel curve to
individual vehicles. Gains of 5-7HP have been reported when the block is used instead of the factory
metering block. The adjustable metering block has the following features:
The factory 350 Holley metering block doesn't have emulsion tube holes drilled in to the main well.
Common practice is to drill different sized holes in the main well chanels to get the engine to operate
at around 12.5:1 air/fuel ratio throughout its power curve. The holes introduce air in different volume
amounts (controlled by the number of bleed holes and
their diameter), and at different points in the fuel curve
(controlled by their vertical placement in the channel).
However, once the holes are drilled, they cannot be
readily “undrilled”. The adjustable metering block has
five screw-in emulsion air bleeds for the main circuit.
These are supplied as 0.031”, 0.031”, 0.031”, 0.020” and
0.020” (bottom to top).
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Similarly, the factory 350 Holley metering block power valve restriction channels (PVCRs) are not
adjustable. Common practice is to drill out the PVCRs larger than the factory size (~0.056”). Again,
once the PVCRs are drilled, they are difficult to “undrill”. The adjustable metering block has screw-in
PVCRs, supplied at 0.042”.
At wide-open throttle (WOT), the high vacuum generated in t
he venturis can suck unmetered fuel from the accelerator
pump discharge nozzles. The adjustable metering block has
an anti-siphon air bleed fitted in the accelerator pump passage
just above the main metering jets (sometimes referred to as a
“kill bleed”). The bleed is supplied with a 0.020” screw-in
restriction. Air introduced into the pump passage by the bleed
breaks the vacuum being pulled on the fuel, and stops the
accelerator pump feeding the mixture at WOT.
A wide range of screw-on restrictions are available to tune the
emulsion air bleeds, PVCRs and kill bleed as per the table below:
Part Number
Hole Size
Part Number
Hole Size
Part Number
Hole Size
Part Number
Hole Size
142-00 Blank 142-29 0.0292” 142-39 0.039” 142-59 0.0595”
142-20 0.020” 142-31 0.031” 142-40 0.040” 142-62 0.0625”
142-21 0.021” 142-32 0.032” 142-41 0.041” 142-64 0.0635”
142-22 0.0225” 142-33 0.033” 142-42 0.042” 142-67 0.067”
142-24 0.024” 142-35 0.035” 142-43 0.043” 142-70 0.070”
142-25 0.025” 142-36 0.036” 142-46 0.0465” 142-73 0.073”
142-26 0.026” 142-37 0.037” 142-52 0.052” 142-76 0.076”
142-28 0.028” 142-38 0.038” 142-55 0.055” 142-78 0.078”
Page 66 of 66
12 Contacts
Holley
Dealers:
http://www.holley.com/dealers/InternationalHolleyDeal
erLocator.asp
Email: http://www.holley.com/TechService/TechRequest.asp
Internet: www.holley.com
AussieSpeed Performance Products
Address: PO Box 4009 Seaton, SA 5023 Australia
Telephone: (04) 03221105
Internet: www.aussiespeed.com
Redline Performance (Hardiman Auto Supplies)
Address: 9 Bullecourt Avenue Milperra, NSW 2214 Australia
Telephone: (02) 87238888
Facsimile: (02) 97712176
Email: [email protected]
Internet: www.redlineauto.com.au
K&N
Dealers: http://www.knfilters.com/search/dealersearch.aspx
Email: [email protected]
Internet: http://www.knfilters.com.au
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