Carburetor Tuning engine

8/20/2019 Carburetor Tuning engine

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1he original e"uipment caruretor(s) that came on a muscle or classic

vehicle:s engine was tuned for the leaded gasoline of the day, so in most

cases the engine will tend to run lean with the reformulated and'or

oygenated unleaded gasoline of today# 1he gasoline of today also has

lower volatility than the leaded gasoline of days past, which will cause

most carureted engines to need a slightly richer +' miture at idle and

light load part throttle driving conditions to have the same drivaility as it

had with the leaded gasoline of the :;0s and :40s#

Bac% in the <0s and early <;0s, the car manufactures tended to

calirate their caruretors on the rich side of the ideal +' miture needs

of the engine with the leaded gasoline of the day# 1hen starting in the late

<;0s, the caruretors were calirated more toward the lean side of the

ideal +' miture needs of the engine so the vehicle could pass the

ehaust emission standards that were =ust coming into eistence#

1he modern reformulated conventional and oygenated gasoline of todaywill cause the +' miture to shift leaner when compared to the leaded

gasoline of the <;0s and <40s# 1his means if the +' miture was lean

with leaded gasoline it will e even leaner with today:s gasoline lends#

1he high performance and replacement caruretors sold today are sold

with an +' miture curve designed for a generic engine6 therefore they

must e tuned for oth the specic engine and the lend of gasoline they

will e used with# 1hese aftermar%et caruretors should e designed with

an +' miture that is rich enough for a wide variety of engine pac%ages

with di/erent ehaust systems, ut this is not always true# >ome of the

aftermar%et caruretors we see need a lot of tuning wor% to get the +'miture correct for the engine:s demands with the reformulated unleaded

gasoline of today#

Air/Fuel Mixture Tuning Guielines

Bac% in the days of leaded gasoline an eperienced tuner would ad=ust the

+' miture the engine was getting from its caruretor y reading the

color the fuel left on the insulator of the spar% plug in the ehaust port and

in the rst ; inches of the ehaust header# 1he reformulated unleaded

gasoline we have today has made reading spar% plugs almost impossile

ecause it leaves little or no color on the spar% plug insulator#

7owever, modern technology has made availale at an a/ordale price

oth portale !gas ehaust gas analy&ers and wideand *amda

(-oygen.) sensor ased digital +' meters that can e used to accurately

-read. the +' miture in an engine y analy&ing the content of the

engine:s ehaust gases# 1hese modern tools can allow you to oserve

what +' miture the engine is getting from the fuel system while driving

the car in real world conditions at any rpm and load condition#

1he ideal +' ratio for maimum power or fuel economy may e estcalculated at the factory with the engine on a dynamometer, ut the



readings that are availale from a !gas ehaust gas analy&er allow you

to tune the +' miture for what your engine needs in real world driving

conditions# 1he readings from an infrared ehaust gas analy&er will

indicate +' ratio, engine misre, engine comustion e?ciency and

ecessive comustion chamer heat (detonation) y loo%ing at the

following ehaust gases5

@A (@aron Monoide)5 1he reading from an infrared gas analy&er that we

use to determine the air to fuel ratio when the +' miture is on the rich

side of stoichiometric# ($ote5 @A is partially urned fuel#)

The other reaings that exhaust analy!ers "ro#ie are:

HC $Hyro%arbons&: 1he amount of unurned fuel in the ehaust (a

indicator of an engine misre)#

CO' $Carbon (ioxie&: + gas that is the product of complete comustion

(the est +' miture gives you the highest @A2 reading)# 1he ideal

ignition!timing advance will also create the highest @A2 reading

O' $Oxygen&: + high A2 reading indicates a lean miture6 an ehaust

lea% or the engine has a -hot. cam# $ote5 if A2 content is aove 2 to 9

percent, air dilution of the ehaust gases eing measured is indicated and

the accuracy of the all of the gas readings may e negatively a/ected#

)Ox $Oxies o* )itrogen&: + gas created y ecessive comustion

chamer heat# 1his gas can e used as a precursor to detonation#

1he readings you can get from a !gas ehaust gas analy&er can help an

eperienced tuner calculate what +' miture and how much ignition spar%

timing advance the engine needs to perform at its est#

+ wideand sensor lamda sensor ased +' meter calculates the +'

miture y -reading. the unurned comustile content of the ehaust

gases (note5 a lot of people call the lamda sensor an oygen sensor ut

Bosch calls it a lamda sensor)# 1he wideand lamda sensor measures

the amount of oygen that must e added to or sutracted from the

ehaust gas to form a stoichiometric gas mi in its reference chamer, the+' meter then calculates the +' miture of the ehaust gas from that

value#

1he readings you get from a wideand lamda sensor ased +' meter

can e "uite accurate, ut false readings can e created y an ehaust

lea%, engine misre, or an engine with a high performance camshaft at

lower engine speeds# 1hese false readings are caused y the *amda

sensor misreading the unused oygen and'or unurned comustiles that

are in the ehaust gas miture



Tuning +ith a ,-Gas Analy!er an ieban a0ba Meter

1he use of a portale !gas ehaust gas analy&er and'or a wideand

sensor ased +' meter can allow a tuner to oserve the +' miture the

engine is getting from its fuel system at any engine operating condition#

+ starting point for +' mitures for most mild performance engines is5

Idle5 #0C to 9#0 C @A or a 3#!9#356

@ruise rpm5 #0C @A or a 3#5 with a mild performance engine6 or

#0C ! 9#0C @A or a 3# D 9#35 with high performance cam6 and

ower miture and acceleration5 ;#0C @A or a 2#5 for a -normal.

engine or high performance engine with improved comustion chamer

design such as a ro >toc% or a $+>@+F engine6 in some cases you may

e ale to use a slightly leaner power miture of 3C @A or a 9#05#

hen we are tuning fuel systems, we use oth infrared ehaust gas

analy&er and the wideand *amda sensor methods# 1his way we can ta%e

advantage of the strengths of oth tuning methods# 1he infrared ehaust

gas analy&er, while slower in reaction time than a wideand sensor ased

+' meter can actually est determine +' miture needs# 1he misre rate

can e oserved with the 7@ (hydrocaron) reading#

E?ciency can e oserved y the @A2 reading (caron dioide) reading,

and the $A reading (oides of nitrogen) can also e used as a precursor

to detonation# + wideand *amda sensor!ased +' meter systems

availale from companies such as Innovate Motorsports or +>1 have

almost no delay, while a !gas ehaust gas analy&er has a ; to 0 second

delay#

If the engine you are tuning has an air!gap style inta%e manifold and'or

high performance camshaft you may need to tune the idle and cruise

mitures richer than a stoc% engine with the same gasoline# 1he added

performance from an air!gap inta%e manifold and the increased valveoverlap from a high performance camshaft can often come at the price of

lower fuel vapori&ation at lower rpm operating conditions#

1he richer +' miture can help cover up the driveaility prolems when

the fuel is not completely vapori&ed# 1he heat the inta%e manifold gets

from the ehaust gas crossover in a conventional inta%e manifold helps

the engine vapori&e the fuel as it travels from the caruretor into the

cylinders comustion chamer#

A/F Mixture (eli#ery Cir%uits



+ caruretor has an accelerator pump, idle, main =ets, and in most cases a

power system that is designed to supply the correct +' miture for the

demands# 1he accelerator pump system adds fuel as the throttle valves

are opened# 1uning the accelerator pump s"uirter volume and duration is

mainly done y trial and error to otain the est throttle response, ut a

2#5 +' miture is a good place to start#

+n idle system will have an idle =et'restriction that must e changed to

supply the desired fuel miture for idle and o/ idle engine demands# If the

engine you are wor%ing on is e"uipped with a power valve (no metering

rods), the main =et si&e is what determines the +' miture that will e

delivered to the engine at light!load'cruise speeds#

1he power valve restriction (under the power valve) determines what +'

miture the caruretor will supply when the power valve is open6 under

high power demands a ;#G power valve will e open, supplying richer +'

miture any time the vacuum is elow its ;#G opening point#

ower valves have a reputation for eing a wea% lin% in certain designs,

ut the caruretor can e retrotted with ac%re protection, which will

improve reliaility# + caruretor that uses metering rods in the primary =ets

will use the metering rods to change the +' ratio for oth the power and

cruise miture demands of the engine6 the larger the metering rod

diameter the leaner the +' miture#

+fter the asic engine condition and tune!up (fuel pressure, timing curve,

etc) is conrmed to e correct, as well as chec%ing to e sure there are no

vacuum lea%s, the net step is to determine what the +' miture is at idlethrough 9,000 rpm# If the cruise miture is o/, rst change the =ets to get

the +' miture correct at 2,00!9,000 rpm cruise range# 1hen chec% and

set the idle miture# If the +' miture is too lean at idle or part throttle

and the idle miture screws do not provide enough ad=ustment, the

correction may involve enlarging the idle =et#

If the miture is still lean at ,000 through ,800 rpm after enlarging the

idle =et, the idle channel restriction (if used) may have to e enlarged

slightly to allow more fuel to e delivered at part throttle# It is important to

note that any changes other than asic ad=ustments and =et changes

should e done y a -caruretor epert. to avoid damaging a vintage

caruretor# If the caruretor is damaged a replacement numers

matching caruretor could e "uite epensive#

+ modular design caruretor, such as a 7olley, with a metering loc% does

not use an idle channel restrictor# hen we want to richen the part throttle

we often must slightly enlarge the idle well in the metering loc%# hen

the +' miture is too lean at part throttle the engine may miss or stumle

on light acceleration and at D 2 mph light throttle cruise conditions#1his

lean o/ idle prolem has ecome more prominent as the ethanol content

in today:s gasoline is increased and as the gasoline formulation ischanged#



If the +' miture is too rich at idle and'or part throttle, the idle =et or part

throttle idle restriction may e too ig# Hou may need to e replace it with

a smaller one# Ance you have the idle, part throttle and cruise +' miture

curves correct, the net step is a road test#

+ road test using a portale infrared ehaust gas analy&er and'or a

wideand oygen sensor will allow you to chec% the cruise speed +'

miture, followed y a chec% of the power +' miture under load# 1his

type of test allows you to see what the +' miture is under real world

driving conditions# uring this road test you will e ale to read and then

correct the +' miture#

If you see an +' miture reading that goes too lean at high engine loads,

the rst thing to do efore you change =et si&e is to chec% the fuel

pressure# 1he fuel pressure must stay aove psi at wide!open throttle6 if

not, the caruretor will starve for fuel#

1he most common accelerator pump!related complaint we hear is a

hesitation on "uic% acceleration# 1his hesitation is most often caused y

the changes in the gasoline:s volatility and changes in caruretor

manufacturing# 1he accelerator pump duration spring used on most

replacement caruretors is not as strong as the spring that was on these

same caruretor designs used in the <;0s#

e use an accelerator pump upgrade %it on most 7olley modular style

caruretors that consists of a stronger duration spring, a 0#09G s"uirterand a -pin% cam. (Ale:s p'n 990), this ma%es the accelerator s"uirt more

active#

hen we are wor%ing on a engine with an Edelroc% erformer or 1hunder

series caruretor we use an improved accelerator pump (Ale:s p'n 00)#

1his accelerator pump has a stronger duration spring that allows the pump

to e more active and thus help cure most of the accelerator pump related

hesitation we see with these caruretors#

1ele%ting the Corre%t Carburetor

1he ig four suppliers of 3 arrel caruretors today are Edelroc%, 7olley,

Juic% uel and Barry Krant, each of these caruretor designs have strong

and wea% spots# 1he caruretor that we would recommend is ased on

how the customer will drive their car and the engine pac%age that is in the

car#

1he @arter!designed Edelroc% 1hunder and erformer are reliale low

maintenance caruretors with great electric cho%es ut if the driver li%es

to drive fast around corners they may not ethe est caruretor to selectfor that application# 1he o/!idle system design in these @arter!designed



caruretors can lead to a lean o/!idle stumle prolem when the engine

has a -hot cam. or an air!gap style inta%e manifold# Enlarging the idle

channel restrictor on the 00 thru ;0 cfm units will often cure this lean

o/!idle stumle prolem ut we have not had the same success solving

this lean o/!idle stumle prolem on the 40 and 800 cfm caruretors of

this design#

Modular caruretors manufactured and sold y companies such as 7olley,

Barry Krant and Juic% uel are very good caruretors to select when the

driver li%es to drive fast around corners or when you are tuning for

maimum power# Juic% uel also sells illet metering loc%s with

changeale idle =ets, power channel restrictors and emulsion well

restrictions for the 7olley style modular caruretors, which allow you to

custom tune the fuel curve#

hen the customer wants a high performance modular caruretor with an

electric cho%e we often recommend a 7olley rand caruretor ecause

their cho%es have a cho%e pull!o/ uilt in# hen we are tuning a high

performance engine with a -hot cam. (over 230 degrees of duration L #00G) or any engine with an air!gap style inta%e manifold, we often

recommend a race!designed modular caruretor with a four corner idle

system#

Tuning 2esults

Hou can uild your customer the perfect engine ut unless it is tuned forthe correct formulation of gasoline the engine will not perform li%e it

should# 1he rst person that most customers lame if the engine does not

perform li%e they thin% it should is the person that uilt the engine# 1he

est way to ensure your customer is satised with the dollars they spent

to reuild their engine is to either o/er a tuning service or give them a

tuning guide#

Ance the engine has een tuned so that it has the correct ignition spar%

timing and the correct +' miture for every operating condition theengine will perform li%e it should# roperly tuned engines will also have

fewer alleged warranty claims since a properly tuned engine will not su/er

from prolems such as the piston rings eing washed down from overly

rich +' mitures, piston damage from detonation or engine tuning related

driveaility prolems#

Carburetor Tuning engine

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