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Common Tuning Issues:

1. Main Jet Size

2. Poor Mid-Range Performance

3. Backfires Through Carburetor 

4. Backfires in Exhaust

5. Detonation ("Spark Knock")

6. Poor Mileage

7. Which Manifold?

We are confident that you will agree with us that your new Mikuni carburetor is one of the best modifications you have made toyour Harley-Davidson motorcycle. The Mikuni substantially increases power compared to the stock carburetor. However it is thedramatic improvement in throttle response that makes the Mikuni flatslide HSR carburetors such outstanding performers. Nocarburetor made today can match the Mikuni in this regard.

We have chosen tuning components that, for the great majority of applications, are correct. Chances are, you have had to dovery little beyond adjusting the air screw and idle speed screw.

However, a change to one engine tuning component often affects or is affected by other components. All of these, the exhaust,air cleaner, ignition, cams, etc., must work together if you are to get the best performance from your Harley engine.

The current series of Harley-Davidson engines share many design and therefore operational features going back at least to theoriginal Evo Big Twin of 1984. We and others have amassed a great deal of information about how various tuning componentswork, work together, do not work together or do not work well at all.

There is no one single combination of parts that is right for all riding styles. The performance requirements of a touring rider usually differ from those of the LTL (Light to Light) performance rider. A combination of performance components that deliversoutstanding power ("Torque") in the middle range (2,000 to 4,000 rpm) seldom does as well above 5,000 rpm. The drag racer usually could care less about what happens at 2,000; his/her concern starts above 4,000 or so. These divergent requirementsare best served by somewhat different combinations of parts. Except, of course, the Mikuni --- they both need that!

It is very difficult to put together successful performance parts combinations without considerable testing and knowledge abouthow these parts work and interact with one another. One can, for instance, fit a camshaft design that favors top-end power andcombine it with an exhaust that restricts top-end power. Such a combination delivers poor performance at both ends of anengine's rpm range. Unfortunately, this sort of mis-match occurs frequently.

Mikuni cannot take the responsibility of specifying all the possible performance parts combinations that work together. However,we may be able to help you avoid the general problem of "wrong" combinations with the information found in the web pagesassigned to the links at the top of this page.

1: Main Jet Size: How to Get it Right

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Mikuni HSR-series carburetors are remarkably versatile instruments. The standard tuningseldom needs more than small adjustments to accommodate a wide range of engine set-ups.One of the more common required changes is the main jet size.

 Aftermarket exhausts have a wide range of flow volumes and the best main jet size is closelyassociated with exhaust flow. Thus, it is often necessary to replace the standard main jet with a

different size to accommodate the wide range of exhaust designs on the market. However, it iseasy to get the main jet right for a particular exhaust system using one of the techniquesdescribed on this page.

The standard main jet fitted to the HSR42 is a number 160. This size is correct for stockmufflers. Typically, an HSR42 combined with aftermarket exhaust system needs a 165 main jet.The general rule is that HSR42s fitted to engines with loud exhausts usually run best with a 165main jet.

The HSR45 has a number 175 and the HSR48 a 190. These jets are more suited to modifiedengines with free flowing exhaust systems.

Keep in mind that the main jet does not affect mixtures until approximately 3/4 throttle. Below

that throttle setting, specifically between 1/4 and 3/4 throttle, air/fuel mixtures are controlled bythe jet needle and needle jet.

It is relatively easy to get the main jet correct. Follow either of the techniques described below.Both are satisfactory but the Roll-On procedure is more accurate.

NOTE:

The following tuning techniques might result in excessive (illegal) speed and increased risk fromthe speed and the necessary distraction of doing the test. We recommend that the testing bedone on a closed course (track) or on a dynamometer, if one is available.

ROLL-OFF:

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First,one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear asthis allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This mayneed to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engineaccelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll thethrottle off to about the 7/8ths position. When you do this, the mixture richens slightly for asecond or so.If the engine gains power as you roll the throttle off, then the main jet is too small and you needto fit a larger one.If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you

need to fit a smaller one.

Copyright (C) 2002, Mikuni American Corporation All Right Reserved.

Carburetor Tuning:

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Typically, mid-range performance is controlled by the jet needle/needle jet combination. This isbecause the majority of mid-rpm operation is at low throttle settings or on the highway atcruising speeds of 50 - 70 mph. The HSR42 or HSR45 can deliver enough air/fuel mixture tosupport these speeds with throttle openings between 1/8th & 1/4, where the straight-diameter part of the jet needle controls fuel flow.

Mikuni supplies four different jet needle sizes to accommodate tuning requirements in thisrange, one set of four for the HSR42, four for the HSR45 and another set for the HSR48. Theydiffer only in the diameter of the straight section of the needle. The leanest is J8-8DDY01-98(HSR42 example part number) and the richest is J8-8CFY02-95 (HSR45 example partnumber). We commonly refer to these needles by their "dash" number (-95, -96, -97 or -98).

Flat throttle response in the mid-rpm range is seldom caused by either an over-rich or overlylean condition. Flat mid-rpm performance is more likely due to the effects of the cam or exhaustdesign. If the needle size is incorrect, it will normally reveal itself as poor mileage (too rich),slow warm-up (too lean) or light detonation when accelerating moderately from around 2500 to2900 rpm (again, too lean).

 A typical FXD (either engine type) motorcycle will deliver around 45 mpg at 65 mph on a flat,

windless road. A heavy touring machine (FLHT- series) may be down a few mpg from thatstandard. Fuel mileage in the 30s indicates a rich condition.

Please refer to the tuning manual, available on the Manuals page for instructions on diagnosingand tuning.

Note:

Confusing symptoms is one of the most common errors in diagnosing carburetor tuninginaccuracies. For instance, low power at 60 mph (2500 rpm) in top gear may have one or moreof several causes: The exhaust system may not work well at that rpm, the cam design may notwork well at that rpm, the ignition timing could be incorrect for that rpm, or, --- the carburetor could be set too lean or too rich at that throttle opening.

Notice that when the carburetor was mentioned above, it is the throttle opening we refer to andnot the rpm. This is an important difference.

While the performance of other engine components depend, to a large extent, upon rpm, thecarburetor only responds to the position of its throttle valve (slide) and the amount of air flowingthrough it (and sometimes the direction of that air flow).

One of the most valuable carburetor tuning aids is to change rpm (down or up shift) whileholding the same road speed. An example: The engine gives poor acceleration from 60 mph(2570 rpm) in top gear. If you maintain the road speed and down shift to fourth gear, the throttlesetting will remain essentially the same but the engine rpm will increase 20%. If the poor topgear acceleration is due to, say, poor exhaust system performance at that rpm, then, the

problem will either go away, get better or at least change its character. If, on the other hand, theproblem is carburetor tuning, the poor acceleration will remain the same because the carburetor throttle opening is the same.

Exhaust system: 

Straight pipes:

Open straight pipes perform poorly in the 2500 to 3800 rpm range. If they are 34" or longer,they do not perform really well at any rpm.

Symptoms include missing, backfiring through the carburetor, reversion (fuel dripping out of theair cleaner) and poor acceleration.

Open mufflers:

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"Gutted" mufflers with stock (or stock-like) header pipes tend to perform poorly in the same rpmrange as straight pipes and exhibit similar symptoms.

Long thin mufflers:

Long, small diameter mufflers with full-length baffles often exhibit the same symptoms asstraight pipes, although their over-all performance may be better.

High performance 2-into-1 systems:

These systems are often poor performers in the 2000 to 3000 rpm range. Most 2-into-1 exhaustsystems deliver a significant torque dip at 2500 which is slightly less than 60 mph in top gear for most stock Harley Big Twins.

Header pipe diameter:

The great majority of Harley engines, of any displacement, do their best work with 1-3/4"diameter exhaust pipes. Larger pipes tend to suppress mid-rpm performance and, for thatmatter, seldom deliver the best power at high rpm either.

Header pipe length:

The stock header pipe is about 30". Multiple tests, made by several groups, confirm this lengthas being very nearly the best for all-round performance. Shorter (less than 27") and longer (over 32") header pipes significantly reduce peak power, throttle response and over-all performance. An exception to this "rule" are a couple of the high performance 2-into-1 systems which workvery well with longer (and un-even) header pipe lengths. Stock Harley header pipes are near-perfect in diameter and length.

Muffler size:

It is not possible to make a muffler quiet, small and powerful at the same time. One can choosepower and small, quiet and small but not all three. The reason stock mufflers are poor performers is because they are small and quiet.

However, small and loud is not a guarantee of performance. In general, small mufflers withlarge straight-through, perforated tube baffles (looks like a tube with many holes drilled in it)make the most power and the most noise. An exception to this rule (there may be more) are thepopular H-D Screamin' Eagle (and Cycle Shack) small slip-on mufflers which perform very wellyet are not straight-through designs. The popular louvered core baffles restrict flow at fullthrottle & high rpm and reduce power a bit as a result.

Too much cam:

The most important cam timing event is when the intake valve closes. The intake closing pointdetermines the minimum rpm at which the engine begins to do its best work. The later theintake valves close, the higher the rpm must be before the engine gets "happy." High rpm cam designs often perform poorly in the rpm range associated with ordinary riding.The problem with such choices is that the engine seldom spends time in the rpm range favoredby such cams. Unfortunately, in the quest for maximum power output, many-too-many Harleyowners choose a late-closing, high-rpm cam for their engine.

 A majority of any Harley motor's life is spent in the mid-portion of is rpm limits, between 2000and 4000 rpm. At open-road cruising speeds, that range is more like 2500 to 3500 rpm. Withcurrent Big Twin gearing, top gear at 2500 rpm returns a road speed of 60 mph and 3500delivers 84 mph. Riders sometimes "putt" around at 2000 or less. Even when accelerating to

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cruising speed, few of us use more than 4000 - 4500 rpm as a shift point. Very seldom, in day-to-day use, do our engines get near 5000 rpm, let alone 6000.

Even the mildest of Harley-Davidson's aftermarket cams (Evo or Twin Cam) do their best workabove 3000 rpm. At 2000, the majority these cams seldom perform as well as the stock cam(s).

The rpm at which a Big Twin gets "happy" can be predicted by the closing point (angle) of theintake valves. The angle is expressed as the number of degrees After Bottom Dead Center (ABDC) that the valves reach .053" from being fully seated.

30 degrees = 2400 rpm35 degrees = 3000 rpm40 degrees = 3600 rpm45 degrees = 4000 rpm50+ degrees = 4500 rpm

These relationships are approximate but should hold true to within 200 rpm or so. They also assume that all other tuningfactors, exhaust, ignition, etc., are operating correctly.

If you have one of the late-closing cam designs installed, say one that closes the intake valves later than 40 degrees, then youcannot expect excellent performance at 2000 rpm. No carburetor adjustment, ignition adjustment or exhaust system canchange this.

Ignition:

Stock H-D Evo Big Twin ignitions have two advance curves ---- a quick advance curve for part-throttle, light load running, and,the very slow advance curve for mid to full-throttle running. It is this second curve that determines the ignition timing whenaccelerating even moderately. While not the most common reason for 'soft' or 'flat' acceleration in the mid-rpm range, the stockEvo ignition doesn't help.

The Screamin' Eagle Evo ignitions have the same full throttle advance curve as the stock ignition. The only difference between

the two is the rev limiter rpm which is 5200 for the stock unit and 8000 (much too high) for the Screamin' Eagle ignition.

Ignitions with quicker advance curves, such as the CompuFire (curves 6,7 or 8) or Dyna 2000 (#1 curve only) have aggressiveadvance curves and improve throttle response and part-throttle performance in the mid-rpm range, especially below 3000 rpm.These two examples are that only; there are other after market ignitions that also contain quicker advance curves.

Stock Twin Cam ignitions are more complex than the earlier Evo type. They use a manifold pressure/engine revolution ratesystem for choosing ignition timing for any combination of rpm and throttle setting. We have no reason to recommend non-Harley ignitions for the Twin Cam engines.

Low compression pressure:

The higher the pressure within the combustion chamber when the air/fuel mixture is ignited, everything else being equal, themore power your engine produces and more efficiently it runs. However, if the pressure it too high, detonation (pinging) mayoccur which can destroy an engine.

Each combustion chamber design has an upper pressure limit above which serious, damaging detonation is likely. With modern American 92 Octane lead-free gasoline, a reasonable upper pressure limit is 180 psi for the Evo Big Twin and 190 psi for theTwin Cam. A well-tuned motor should not suffer detonation with these pressures.

The standard method for determining the compression or cranking pressure of an engine is to remove the spark plugs, install astandard compression gauge into one of the spark plug holes and, with the throttle full-open, crank the engine over with thestarter motor until the pressure gauge needle stops rising. This usually takes 4 - 8 compression strokes. Both cylinders shouldbe tested.

Stock Evo and Twin Cam motors develop cranking pressures in the 150 psi range. If a late-closing cam is installed, with noother changes, the cranking pressure will go down. The reason high compression ratio pistons and racing cams are so oftenassociated is because the higher compression ratio pistons (and/or milled heads) are needed to regain even the normalmoderate cranking pressures, let alone raise them for more power and efficiency.

Low cranking pressures (because of late closing cams and stock pistons) can significantly reduce performance in the mid-rpmrange. 

Copyright (C) 2002, Mikuni American Corporation All Right Reserved.

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4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when

the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned highperformance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closedand the engine is rotating well above idle speed. It is also necessary that the exhaust systemhave rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system(needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strongvacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the

small gap formed by the throttle valve and carburetor throat.3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal,

combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustionchamber. It gets sent into the exhaust system unburned and collects there.

4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaustsystem where it sometimes ignites the raw mixture that has collected ---- the exhaust thenpops or backfires.

5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have anopen exit for the popping to occur.

Other possible causes:

Air Leaks:

 Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. Themost common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase theintensity or likelihood of exhaust system backfiring.

 A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before lookingfor other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignitionsystem is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust poppingcan become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switchesor connectors as possible causes. 

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5: Detonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosions that takeplace within an engine's combustion chambers. It can be extremely destructive, breakingpistons, rod bearings and anything else from the pistons down that a large hammer could

damage. It is best avoided.

Pinging is a descriptive name for detonation. Pinging is that high pitch ringing sound that anengine sometimes makes when the throttle is opened with the engine under load. It sounds asthough the cooling fins are ringing as they do when you quickly run your finger nail over their edges.

Pinging indicates trouble. Trouble that does damage. That damage can be quick andcatastrophic but usually isn't. Most often, detonation occurances are small in energy and theengine is able to absorb the punishment, at least temporarily. However, over time, even lightdetonation does harm; weakening pistons and overheating the top piston rings.

Severe detonation can destroy an engine literally in a heart beat.

HOW IT HAPPENS

 After a spark ignites the air/fuel mixture in an engine's combustion chamber, the flame fronttravels across the chamber at a rate of about 5000 feet per second. That's right, one mile per second.

Flame front travel for detonation is closer to 19,000 to 25,000 feet per second; the same rate asin dynamite. The difference between normal combustion and detonation is the rate at which theburning takes place and therefore the rate of pressure rise in the chamber. The hammer likeblows of detonation literally ring the metal structures of the motor and that is what you hear aspinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal burning has theflame front traveling from the spark plug(s) across the chamber in a predictable way. Peakchamber pressure occurs at about 12 degrees after top dead center and the piston gets pusheddown the bore.

Sometimes and for various reasons a second flame front starts across the chamber from theoriginal source of ignition. The chamber pressure then rises too rapidly for piston movement torelieve it. The pressure and temperature become so great that all the mixture in the chamber explodes. If the force of that explosion is great enough --- the engine breaks.

WHAT CAUSES IT

 Anytime the combustion chamber pressures become high enough, detonation occurs. Anythingthat creates such pressure is the cause of detonation.

Here is a list of possible causes, it may not be complete:

* Timing - if the spark happens too soon, the chamber pressure may rise too high anddetonation results.* Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure anddetonation are likely.* Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can startburning too soon. Pressure rises too high and detonation can happen.* Cranking pressure - Any given combustion chamber has a maximum pressure (before thespark is struck) beyond which detonation is likely.* High engine tempertures - High chamber temperatures raise cranking pressure and promotedetonation.* Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber,

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uneven burning, pressure spikes and detonation.

Note that each of these possible causes are relative. That is, there is no absolute timing,mixture strength or ignition timing that is going to guarantee detonation. Equally, there are noabsolute settings that guarantee that detonation does not occur.

Motorcycle manufacturers, Harley-Davidson included, spend a great deal of time and moneyfine tuning their engines to eliminate or nearly eliminate detonation. When we change theengine design in the direction of detonation by, say, raising the compression pressure withdomed pistons or milled heads, we increase the chance of detonation actually occurring.

Gasoline quality helps determine whether or not an engine is going to detonate. The higher theoctane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase thelikelyhood of detonation. High compression pistons, thin head gaskets, some alternativeignitions, some exhaust system designs, etc.

Stock street bike carburetion is very lean for emissions purposes. When the air cleaner and/or 

exhaust system are replaced by less restrictive components, this stock jetting becomesimpossibly lean. The engine does not run well and detonation is likely at some throttle settings.Re-jetting or wholesale carburetor replacement (Mikuni!) is the cure for this particular problem.

If one fits high compression ratio pistons together with an early closing (mild) cam, the crankingpressure may become high enough that serious, engine-deadly detonation is likely. How muchis too much you ask?

Well (Rule of Thumb here), Evolution engines are fairly safe against detonation if the crankingpressure remains at 180 psi or less. The TC88 motor can dodge detonation if the pressuresremain at 190 psi or less. Keep in mind that these maximums are for fairly stock engines; noporting, no chamber work and no squish areas. A well shaped combustion chamber with squish effect is much less likely to detonate than most

stock examples. The main reason the TC88 engine can withstand higher cranking pressuresthan the Evo is its better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compressiontest. This test is done by removing the spark plugs and fitting a compression gage in one of thespark plug holes. The throttle is then held open and the engine cranked with the starter until thegage needle stops climbing. The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure mayrise too quickly bringing on detonation. The main reason for having an advance curve built intoan ignition system is to avoid detonation. The correct timing for any given engine design (andstate of tune) varies with rpm and throttle setting.

Hot spots is more than a night club. If your engine has been running rich or burning oil, it mayhave thick bits of burned-on carbon. This carbon build-up can literally glow and, under thepressure of compression, start burning before the spark is struck. This leads to severe pressureexcursions and, often, detonation.Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures canescalate pressures and bring about sudden explosive burning. Also, lean mixtures elevatechamber temperatures which, as you now know, can lead to dreaded detonation.If all this leads you to think that your engine is in imminent peril, then we have succeeded.Detonation is a terrible thing to happen to your expensive Harley engine. The pressures of those explosive events can be enough to hammer rod bearings, pistons and rings into useless junk.

If you hear the tell-tale ringing of detonation next time you open the throttle on a hot day or at

low rpm or after a tank of questionable gasoline, back off the throttle and ride carefully until youcan find and render harmless this demon visiting destruction upon your motor.

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6: Poor Mileage

"Normal" fuel mileage normally varies somewhat depending upon a number of factors. Anaverage range for an FXD-series Harley is: 45-51 miles per gallon at 65 mph on a flat road withno wind. The large touring models typically deliver about five miles per gallon less. Fuel mileageof less than 40 mpg at a steady 65 mph (flat road, no wind) indicates a possible mechanical or tuning problem.

Common causes:

Choke cable installation: An incorrectly installed choke cable can lead to poor fuelmileage.

Carburetor tuning: An incorrectly jetted carburetor can lead to both poor fuelmileage and performance.

Speed: Fuel consumption increases dramatically with speed.

Head wind: Fuel consumption increases when riding into the wind.

Weight: Motorcycles require more fuel when climbing.

Size: Larger (touring) models create more wind drag.

Engine efficiency:Highly developed engines use fuel more efficiently. Poorlytuned ones do not.

Choke cable installation:

There must be some free play in the choke cable to ensure that the starter (choke) plunger is fully closed. If the choke is held

even slightly open, poor mileage, sluggish performance and fouled spark plugs may result.

Harley choke cable

If you are using the Harley choke cable (the word ìChokeî on the knob is white), use this procedure to determine if the choke isclosing completely:

• Pull the choke knob out fully. • Loosen the friction nut just enough to allow the choke shaft to move freely. The friction nut is located behind the

choke knob. It is thin and has ridges around its outer edge like a coin. If you turn the friction nut out too far, it willinterfere with your ability to detect free play in the choke. 

• Now, move the choke knob in fully. Gently pull the knob out. There should be a small amount of free play before you

feel the tension of the choke return spring. If there is no free play:

Check the routing of the cable. The stock Harley cable is very stiff and tends to bind in the metal elbow at the carburetor-end of the cable. The end of the cable slips into the metal elbow and can jamb. The joint (cable/elbow) is hidden by a rubber cover.Push the cable end fully into the elbow.

• If this does not cure the problem, it is possible that the choke cable assembly was not assembled correctly. You must

use the Mikuni choke plunger and spring with the Harley choke cable. If you install the complete Harley assembly(cable, plunger and spring), the Harley plunger will not seal and the air/fuel mixture will be very rich, especially at idleand low throttle settings. 

Mikuni choke cable:

The Mikuni choke cable is identified by the small brass bump in the center of the knob. Mikuniís cable is much more flexible that

the stock Harley cable and seldom jambs. However, it is possible that its length adjustment can be incorrect in a particular installation.

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• Check for free play by gently pulling the knob. It should move freely for a short distance before the force of the

return spring is felt. Even a slight amount of free play is enough. • If there is no free play, check the routing of the cable to make sure that it is not kinked or pinched by other 

components. If necessary, peel the rubber cover back and adjust the length of the cable to introduce a smallamount of free play. 

Carburetor tuning:

Mikuni HSR42/45/48 carburetors are jetted to meet the requirements of the great majority of engine tuning setups. TheHSR-series is very tolerant of engine tuning variations. However, it is certainly possible that minor tuning adjustments maybe desirable to achieve maximum performance and/or maximum fuel economy with some engine componentcombinations. Normal highway cruising speeds (65 mph/ 100 kph) require rather low throttle openings, generally less than º throttle.

 Air/fuel ratios in this throttle range are controlled by the pilot circuit together with the jet needle and needle jet. Thus, poor fuel economy at normal cruising speeds should be addressed by altering or adjusting these parts.

Pilot system:

The pilot circuit has one replacable part and one adjustable part.The pilot jet is replacable and the pilot air screw isadjustable. If the jet is too large or the air screw is in too far, the air/fuel mixture may be too rich. However, it is veryunlikely that the pilot jets installed at the Mikuni factory (#20 or #25) can cause a dramatic loss of fuel economy. SomeHSR45s are fitted with #35 pilot jets and these may be too rich for well-tuned engines. See the tuning manual elsewhereon this website.

Jet needle:

There are four different jet needles. Their part numbers are: J8-8DDY01-95, -96, -97 and ñ98 (J8-8CFY02-xx for the 45 &48). We commonly refer to them as a ìdash 97, dash 98, etc.î The current standard jet needle is a ì-97.î 

The only difference between each needle is the diameter of the straight part of the needle. This is the portion of the needlethat controls air/fuel mixture strengths between idle and approximately º throttle. So, when a mixture change needs to bemade in this range, it is necessary to exchange jet needles. Raising or lowering the needle has no effect on mixturesbelow º throttle.

The jet needle is both adjustable and replacable. Its height can be adjusted (via an ìEî clip and five grooves) to changemixture strength between º and æ throttle. Lowering the jet needle leans the mixture and raising it richens the mixture.

Main jet:

The main jet becomes the main fuel control at approximately 3/4 throttle. The main jet has no effect on fuel mileage under 

any but the most extreme riding conditions.

Speed: 

Fuel consumption increases dramatically with speed. For instance: If you wish to double the speed, your engine mustproduce approximately eight times as much power. Thus, if 20 horsepower gets you 100 miles per hour, you'll need 160HP to go 200. From this relationship it is easy to understand why fuel economy drops so dramatically between normalcrusing speeds (65 - 75 miles per hour in America) and higher speeds around or above 85 mph.

Head wind:

Fuel consumption increases when riding into the wind. If you ride at 60 mph with a 20 mph headwind the fuel mileage is bebetter than if you were going 80 mph with no head wind. However, there is still a significant loss of fuel economy.

Weight:

Heavy motorcycles require more fuel when climbing. This is simple to understand; the more weight lifted, the more energy (fuel)

needed to lift it. Thus, a 20 percent heavier motorcycle requires about 20 percent more fuel to climb a mountain at a givenspeed than the lighter machine.

 An engine that has been modified to perform best in a higher-than-normal rpm range may suffer a dramatic loss of fuel economyif it is operated under load at an engine speed below its design minimum.

Size:

Larger (touring) models create more wind drag.

Engine efficiency:

Highly developed engines use fuel more efficiently. Poorly tuned ones do not.  

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