Crazy Horse Carb School

8/7/2019 Crazy Horse Carb School

1/61

www.crazyhorsemotorcycles.com

CRAZY HORSE MOTORCYCLES

Carburetor Class


2/61


Mikuni Motorcycle

Carburetor TheoryMotorcycle carburetors look very complex, but with a little theory, you can tune your bike formaximum performance.

All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressureis a powerful force which exerts pressure on everything. It varies slightly but is generally

considered to be 15 pounds per square inch (PSI).

This means that atmospheric pressure is pressing on everything at 15 PSI. By varying theatmospheric pressure inside the engine and carburetor, we can change the pressure andmake fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston goes

down on a four stroke engine a low pressure is formed above the piston on a four stroke.

This low pressure also causes a low pressure inside the carburetor. Since the pressure ishigher outside the engine and carburetor, air will rush inside the carburetor and engine untilthe pressure is equalized.

The moving air going through the carburetor will pick up fuel and mix with the air.


3/61


Cycles


4/61


Engine

layout


5/61


Inside a carburetor is a Venturi. The Venturi is a restriction inside thecarburetor that forces air to speed up to get through. A river that suddenlynarrows can be used to illustrate what happens inside a carb. The water inthe river speeds up as it gets near the narrowed shores and will get faster ifthe river narrows even more. The same thing happens inside the carburetor.The air that is speeding up will cause atmospheric pressure to drop insidethe carburetor. The faster the air moves, the lower the pressure inside thecarburetor.


6/61


Carburetor Circuits

Most motorcycle carburetor circuits are governed by throttle

position and not by engine speed.

There are five main metering systems inside most motorcycle

carburetors. These metering circuits overlap each other and

they are:

* pilot circuit

* throttle valve

* needle jet and jet needle

* main jet

* choke circuit


7/61


Drawing

of Carb


8/61


View of

Carb


9/61


Pilot CircuitThe pilot circuit has two adjustable parts. The pilot air screw

and pilot jet. The air screw can be located either near theback side of the carburetor or near the front of the

carburetor.

If the screw is located near the back, it regulates how much

airenters the circuit. If the screw is turned in, it reduces the

amount of air and richens the mixture.

If it is turned out, it opens the passage more and allows

more air into the circuit which results in a lean mixture. Ifthe screw is located near the front, it regulates fuel.

The mixture will be leaner if it is screwed in and richer if

screwed out. If the air screw has to be turned more than 2

turns out for best idling, the next smaller size pilot jet will be

needed.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has

a small hole in it which restricts fuel flow though it. Both the pilot air screw and

pilot jet affects carburetion from idle to around 1/4 throttle.


10/61


Slide ValveThe slide valve affects carburetion

between 1/8 thru 1/2 throttle. It

especially affects it between 1/8 and

1/4 and has a lesser affect up to 1/2.

The slides come in various sizes and

the size is determined by how much

is cutaway from the backside of it, fig

3. The larger the cutaway, the leanerthe mixture (since more air is allowed

through it) and the smaller the

cutaway, the richer the mixture will

be. Throttle valves have numbers on

them that explains how much the

cutaway is. If there is a 3 stampedinto the slide, it has a 3.0mm

cutaway, while a 1 will have a 1.0mm

cutaway

(which will be richer than a 3).


11/61


Jet Needle

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jetneedle is a long tapered rod that controls how much fuel can be drawn into the

carburetor venturi.

The thinner the taper, the richer the mixture. The thicker the taper, the leaner the

mixture since the thicker taper will not allow as much fuel into the venturi as a leaner

one. The tapers are designed very precisely to give different mixtures at different

throttle openings. Jet needles have grooves cut into the top.

A clip goes into one of these grooves and holds it from falling or moving from the

slide. The clip position can be changed to make an engine run richer or leaner, fig 4.

If the engine needs to run leaner, the clip would be moved higher. This will drop the

needle farther down into the needle jet and cause less fuel to flow past it. If the clip is

lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameterof the needle jet, it will affect the jet needle. The needle jet and jet needle work

together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning

for this range is done to the jet needle, and not the needle jet.


12/61


Jet Needle


13/61


Main Jet

Once the throttle is opened far enough, the jet needle is pulled high enough out

of the needle jet and the size of the hole in the main jet begins to regulate fuelflow. Main jets have different size holes in them and the bigger the hole, the more

fuel that will flow (and the richer the mixture). The higher the number on the main

jet, the more fuel that can flow through it and the richer the mixture.

The main jet controls fuel flow from 3/4

thru full throttle.


14/61


Choke

The choke system is used to start cold engines.

Since the fuel in a cold engine is sticking to the cylinder

walls due to condensation, the mixture is too lean for the

engine to start.The choke system will add fuel to the engine to compensate

for the fuel that is stuck to the cylinder walls.

Once the engine is warmed up, condensation is not a

problem, and the choke is not needed.


15/61


Air/Fuel Mixture

The air/fuel mixture must bechanges to meet the demands

of the needs of the engine. The

ideal air/fuel ratio is 14.7

grams of air to 1 gram of fuel.

This ideal ratio is only

achieved for a very shortperiod while the engine is

running. Due to the incomplete

vaporization of fuel at slow

speeds or the additional fuel

required at high speeds, the

actual operational air/fuel ratiois usually richer.

The drawing to the right

shows the actual air/fuel ratio

for any given throttle opening.


16/61


Working Range


17/61


Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known.

The first step is to find where the engine is running poorly.

It must be remembered that carburetor jetting is determined by the throttle

position, not engine speed.

If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or

slide valve is the likely problem.

If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle

jet (most likely the jet needle) is likely the problem.

If the engine is running poorly at 3/4 to full throttle, the main jet is the likely

problem.


18/61


Preparing to TuneWhile jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the

throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other.

When these two lines are lined up, the engine will be idling.

Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At

this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-

way point between both of the lines on the throttle housing.

Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until

idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact

throttle opening while jetting.


19/61


Start TuningClean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a

slight uphill is the best place for this).

After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the

engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan

color (for more info on reading spark plugs click here). If it's white, the air/fuel mixture is too lean and a

bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a

smaller main jet will have to be installed. While changing jets, change them one size at a time, test run

after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the

clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the

air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly

just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the

back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it.

If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between

one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or

smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between

adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.


20/61


Altitude, Humidly, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors thatwill change the performance of the engine.

Altitude, air temperature, and humidity are big factors that will affect how an

engine will run. Air density increases as air gets colder. This means that there are

more oxygen molecules in the same space when the air is cold.

When the temperature drops, the engine will run leaner and more fuel will have to

be added to compensate. When the air temperature gets warmer, the engine will

run richer and less fuel will be needed. An engine that is jetted at 32deg

Fahrenheit may run poorly when the temperature reaches 90deg Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases.

A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.


21/61


Correction factorsHumidity is how much moister is in the air. As humidity increases, jetting will be

richer. A bike that runs fins in the mornings dry air may run rich as the day goes onand the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for

changing temperatures and altitudes.

The chart in fig 8, shows a typical correction factor chart. To use this chart, jet thecarburetor and write down the pilot and main jet sizes. Determine the correct air

temperature and follow the chart over to the right until the correct elevation is

found. Move straight down from this point until the correct correction factor is

found.

Using fig 8 as an example, the air temperature is 95deg Fahrenheit and thealtitude is 3200 ft. The correction factor will be 0.92. To find out the correction main

and pilot jets, multiple the correction factor and each jet size. A main jet size of 350

would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet

size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.


22/61


Correction factors


23/61


Correction factorsCorrection factors can also be used to find the correct settings for the needle jet,

jet needle, and air screw. Use the chart and determine the correction factor. Then

use the table below to determine what to do with the needle jet, jet needle, and

air screw.


24/61


Roll Off Method

1: Main Jet Size: How to Get it Right

Mikuni HSR-series carburetors are remarkably versatile instruments. The standard

tuning seldom 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 sizeis closely associated 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 is easy to get the main jet right for a particular

exhaust system using one of the techniques described on this page.

The standard main jet fitted to the HSR42 is a number 160. This size is correct forstock mufflers. 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 165 main jet.

The HSR45 has a number 175 and the HSR48 a 190. These jets are more suited to

modified engines with free flowing exhaust systems.


25/61


Roll Off Method

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 by the 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 moreaccurate.

NOTE:

The following tuning techniques might result in excessive (illegal) speed and

increased risk from the speed and the necessary distraction of doing the test. Werecommend that the testing be done on a closed course (track) or on a

dynamometer, if one is available.


26/61


Roll Off Method

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 as this 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 may need to be as high as 4000 rpm with some cam choices. Apply full

throttle. Let the engine accelerate for a couple of seconds until it has settled in

and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When

you do this, the mixture richens slightly for a second or so.

If the engine gains power as you roll the throttle off, then the main jet is too small

and you need to 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.


27/61


Poor Mid-Range Performance2: Poor Mid-Range Performance

Possible Causes:

1.Carburetor Tuning

2.Exhaust system

3.Too much cam4.Ignition

5.Low compression pressure

Carburetor Tuning:

Typically, mid-range performance is controlled by the jet needle/needle jetcombination. This is because the majority of mid-rpm operation is at low throttle

settings or on the highway at cruising speeds of 50 - 70 mph. The HSR42 or

HSR45 can deliver enough air/fuel mixture to support these speeds with throttle

openings between 1/8th & 1/4, where the straight-diameter part of the jet needle

controls fuel flow.


28/61


Mid-Ranged PerformanceMikuni supplies four different jet needle sizes to accommodate tuning

requirements in this range, one set of four for the HSR42, four for the HSR45 and

another set for the HSR48. They differ 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 part number). 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 overly lean condition. Flat mid-rpm performance is more likely due to the

effects of the cam or exhaust design. 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 to 2900 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 that standard. Fuel mileage in the 30s indicates a rich

condition.


29/61


Mid-Range PerformancePlease refer to the tuning manual, available on the Manuals page for instructions

on diagnosing and tuning.

Note:

Confusing symptoms is one of the most common errors in diagnosing carburetor

tuning inaccuracies. For instance, low power at 60 mph (2500 rpm) in top gear

may have one or more of several causes: The exhaust system may not work wellat that rpm, the cam design may not work 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 and not the rpm. This is an important difference.

While the performance of other engine components depend, to a large extent,

upon rpm, the carburetor only responds to the position of its throttle valve (slide)

and the amount of air flowing through it (and sometimes the direction of that air

flow).


30/61


Mid-Range

One of the most valuable carburetor tuning aids is to change rpm (down or up

shift) while holding 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 throttle setting will remain essentially the same

but the engine rpm will increase 20%. If the poor top gear acceleration is due to,

say, poor exhaust system performance at that rpm, then, the problem will either goaway, get better or at least change its character. If, on the other hand, the problem

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.


31/61


Mid-Range

Symptoms include missing, backfiring through the carburetor, reversion (fuel

dripping out of the air cleaner) and poor acceleration.

Open mufflers:

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

Long thin mufflers:

Long, small diameter mufflers with full-length baffles often exhibit the same

symptoms as straight 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 exhaust systems 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:


32/61


Mid-RangeThe 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 performanceand, for that matter, 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 length as being very nearly the best for all-round performance. Shorter (lessthan 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 work very 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 choose power and small, quiet and small but not all three. The reason stock

mufflers are poor performers is because they are small and quiet.


33/61


Mid-RangeHowever, small and loud is not a guarantee of performance. In general, small

mufflers with large 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 the popular H-D Screamin' Eagle (and Cycle

Shack) small slip-on mufflers which perform very well yet are not straight-through

designs. The popular louvered core baffles restrict flow at full throttle & 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 point determines the minimum rpm at which the engine begins to do its best

work. The later the intake 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 favored by such cams. Unfortunately, in the quest for

maximum power output, many-too-many Harley owners choose a late-closing, high-

rpm cam for their engine.


34/61


Mid-Range

A majority of any Harley motor's life is spent in the mid-portion of is rpm limits, between 2000

and 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 3500

delivers 84 mph.

Riders sometimes "putt" around at 2000 or less. Even when accelerating to 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 bestwork above 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

the intake 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 rpm




50+ degrees = 4500 rpm


35/61


Mid-RangeThese relationships are approximate but should hold true to within 200 rpm or so. They also

assume that all other tuning factors, 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 you cannot expect excellent performance at 2000 rpm. No

carburetor adjustment, ignition adjustment or exhaust system can change 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 when accelerating even

moderately.

While not the most common reason for 'soft' or 'flat' acceleration in the mid-rpm range, the

stock Evo 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.


36/61


Mid-RangeIgnitions with quicker advance curves, such as the CompuFire (curves 6,7 or 8) or Dyna 2000 (#1 curve only)

have aggressive advance 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 rate system 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, the more power your engine produces and more efficiently it runs. However, if the pressure it too

high, detonation (pinging) may occur 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 the Twin Cam. A well-tuned motor should not suffer detonation with these

pressures.


37/61


Mid-Range

The standard method for determining the compression or cranking pressure of anengine is to remove the spark plugs, install a standard compression gauge into one

of the spark plug holes and, with the throttle full-open, crank the engine over with

the starter motor until the pressure gauge needle stops rising. This usually takes 4

- 8 compression strokes. Both cylinders should be tested.

Stock Evo and Twin Cam motors develop cranking pressures in the 150 psi range.If a late-closing cam is installed, with no other changes, the cranking pressure will

go down. The reason high compression ratio pistons and racing cams are so often

associated is because the higher compression ratio pistons (and/or milled heads)

are needed to regain even the normal moderate 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-rpm range.


38/61


BackfireBackfires Through Carburetor

Common Causes:

Ignition: The factory Evolution engine's ignition can contribute backfiring through

the carburetor.

Cam design: Long duration cams with early opening intake valves can contribute to

backfiring.

Intake manifold air leak: A lean condition due to an intake manifold air leak can

cause backfiring.

Carburetor jetting: An overly-lean low-speed circuit, non-functioning accelerator

pump or clogged pilot jet can contribute to backfiring.


39/61


Backfire

Ignition:

Harley ignition systems have been dual fire for decades. Virtually all stock

Evolution engines, Big Twin & Sportster, have dual fire ignitions. The PP100 used

in the Gilroy era Indians came stock with duel fire. The exceptions are the EFItouring bikes and the 98 & later Sportster Sport models. All Twin Cam engines are

fitted with single fire ignitions. Under normal conditions dual fire ignitions present

no problems. However, when combined with high performance long duration cams

the stock ignition can cause premature ignition of an air/fuel mixture entering the

rear cylinder. This, in turn, results in backfiring through the open intake valve into

the intake system.


40/61


BackfireDual fire ignitions fire front and rear cylinder spark plugs together. One of the sparks

starts combustion while the other is wasted in other cylinder which is not on its firingstroke.

When the rear cylinder is getting a useful spark, the front cylinder s spark is

occurring near the middle of its exhaust stroke. There is nothing to burn in the front

cylinder at this time.

However, when the front cylinder is getting its useful spark, the rear cylinder is on its

intake stroke and a combustible mixture may be present. If that mixture is ignited by

the wasted spark, then a backfire occurs as the burning mixture forces its way

past the intake valve and out through the intake manifold and carburetor.

Single fire ignitions can often eliminate carburetor backfiring since they do notproduce a wasted spark in the rear cylinder. In fact, single fire ignitions can

generally eliminate backfiring in any Harley. For instance, EFI and Twin Cam

engines very seldom backfire through their intakes; both have single fire ignition

systems.


41/61


BackfireCam design:

The earlier the intake valve opens the more likely the dual fire ignition will ignite

air/fuel mixture in the rear cylinder. High performance long duration cams open the

intake valves earlier than the stock one. This is the main reason why modified

Harley engines tend to backfire through the carburetor more frequently than stock

engines.

Intake manifold air leak:

A common and continuing problem with Harley engines is air leaks around the

junction of the manifold and the cylinder heads. Carburetor/manifold leaks are

much less common. An air leak can cause carburetor backfiring.

Other symptoms of an air leak include a slow return to idle or an irregular idle.


42/61


BackfireCarburetor jetting:

Excessively lean carburetor settings can contribute to backfiring. If the mixture is

too lean, it may burn very slowly and unevenly. This condition, in turn, may

result in burning mixture remaining in the cylinder until the beginning of the next

intake stroke when it can ignite the incoming air/fuel mixture.

A too-small or partially blocked pilot jet can bring about this condition.

An accelerator pump adjustment that starts the pump too late can cause this

problem.

A partial vacuum in the fuel tank can reduce fuel flow and bring about a lean

condition. The common factory Harley gas cap that incorporates a one-wayvalve (for emission purposes) sometimes restricts air flow into the tank. This

restriction can result in a partial vacuum and fuel flow restriction.


43/61


Backfire through ExhaustBackfires 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 high performance engine to "pop" and "crackle" when the throttle isclosed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle

is closed and the engine is rotating well above idle speed. It is also necessary that

the exhaust system have rather open mufflers.


44/61


Backfire-ExhaustWhy 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 strong vacuum in the intake manifold. This vacuum, in turn,

causes a high air flow rate through the small gap formed by the throttle valve andcarburetor 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 combustion chamber. 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 exhaust system where it sometimes ignites

the raw mixture that has collected ---- the exhaust then pops orbackfires.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 an open exit for the popping to occur.


45/61


Backfire-ExhaustOther possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions

that bring about exhaust backfiring. The most 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.


46/61


Backfire-Exhaust

Be sure that your carburetor's pilot jet is the correct size and that the idle air

mixture screw is correctly adjusted before looking for 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 ignition system is not performing as it

should. If, for some reason, the ignition sometimes fires at the wrong time, then

exhaust popping can become very energetic (loud). Look for failing high tension

leads (plug wires), failing ignition coil(s) and especially switches or connectors as

possible causes.


47/61


Spark KnockDetonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosionsthat take place within an engine's combustion chambers. It can be extremely

destructive, breaking pistons, 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 an engine sometimes makes when the throttle is opened with theengine under load. It sounds as though 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

and catastrophic but usually isn't. Most often, detonation occurrences are small in

energy and the engine is able to absorb the punishment, at least temporarily.However, over time, even light detonation does harm; weakening pistons and

overheating the top piston rings.

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


48/61


Spark KnockHOW IT HAPPENS

After a spark ignites the air/fuel mixture in an engine's combustion chamber, the

flame front travels 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 as in dynamite. The difference between normal combustion anddetonation is the rate at which the burning takes place and therefore the rate of

pressure rise in the chamber. The hammer like blows of detonation literally ring the

metal structures of the motor and that is what you hear as pinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal

burning has the flame front traveling from the spark plug(s) across the chamber ina predictable way. Peak chamber pressure occurs at about 12 degrees after top

dead center and the piston gets pushed down the bore.


49/61


Spark Knock

Sometimes and for various reasons a second flame front

starts across the chamber from the original source of ignition.

The chamber pressure then rises too rapidly for pistonmovement to relieve 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.


50/61


Spark KnockWHAT CAUSES ITAnytime the combustion chamber pressures become high enough, detonation occurs. Anything that

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 and detonation results.

Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure and detonation are

likely.

Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can start burning toosoon. Pressure rises too high and detonation can happen.

Cranking pressure - Any given combustion chamber has a maximum pressure (before the spark is

struck) beyond which detonation is likely.

High engine temperatures - High chamber temperatures raise cranking pressure and promote

detonation.

Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber, 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 no absolute settings that

guarantee that detonation does not occur.


51/61


Spark KnockMotorcycle manufacturers, Harley-Davidson included, spend a great deal of time and money

fine 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 with

domed 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 the

octane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase thelikelihood of detonation. High compression pistons, thin head gaskets, some alternative

ignitions, 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 becomes

impossibly 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

cranking pressure may become high enough that serious, engine-deadly detonation is likely.

How much is too much you ask?


52/61


Spark Knock

Well (Rule of Thumb here), Evolution engines are fairly safe against detonation if the cranking pressure

remains at 180 psi or less. The TC88 motor can dodge detonation if the pressures remain at 190 psi orless. Keep in mind that these maximums are for fairly stock engines; no porting, 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 pressures than the Evo is its

better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compression test. This testis done by removing the spark plugs and fitting a compression gage in one of the spark plug holes.

The throttle is then held open and the engine cranked with the starter until the gage needle stops climbing.

The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure may rise too

quickly bringing on detonation. The main reason for having an advance curve built into an ignition system

is to avoid detonation. The correct timing for any given engine design (and state 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 may have thick

bits of burned-on carbon. This carbon build-up can literally glow and, under the pressure of compression,

start burning before the spark is struck. This leads to severe pressure excursions and, often, detonation.


53/61


Spark Knock

Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures can

escalate pressures and bring about sudden explosive burning. Also, lean mixtures elevate

chamber 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

you can find and render harmless this demon visiting destruction upon your motor.


54/61


Poor Mileage

6: Poor Mileage

"Normal" fuel mileage normally varies somewhat depending upon a number of factors. An

average range for an FXD-series Harley is: 45-51 miles per gallon at 65 mph on a flat road

with no wind. The large touring models typically deliver about five miles per gallon less. Fuel

mileage of 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 fuel mileage.

Carburetor tuning: An incorrectly jetted carburetor can lead to both poor fuel mileage 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. Poorly tuned ones do

not.


55/61


Poor MileageChoke 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 Choke on the knob is white), use thisprocedure to determine if the choke is closing 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 will interfere 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 amountof free play before you feel the tension of the choke return spring.


56/61


Poor MileageIf there is no free play:

Check the routing of the cable. The stock Harley cable is very stiff and tends to bind in themetal 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 idle andlow throttle settings.

Mikuni choke cable:

The Mikuni choke cable is identified by the small brass bump in the center of the knob.

Mikunis 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.

Check for free play by gently pulling the knob. It should move freely for a short distancebefore 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 small amount of free play.


57/61


Poor MileageCarburetor tuning:

Mikuni HSR42/45/48 carburetors are jetted to meet the requirements of the great majority of

engine tuning setups. The HSR-series is very tolerant of engine tuning variations. However, it

is certainly possible that minor tuning adjustments may be desirable to achieve maximum

performance and/or maximum fuel economy with some engine component combinations.

Normal highway cruising speeds (65 mph/ 100 kph) require rather low throttle openings,

generally less than throttle. Air/fuel ratios in this throttle range are controlled by the pilotcircuit 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 replaceable part and one adjustable part. The pilot jet is replaceable

and the pilot air screw is adjustable. If the jet is too large or the air screw is in too far, theair/fuel mixture may be too rich. However, it is very unlikely that the pilot jets installed at the

Mikuni factory (#20 or #25) can cause a dramatic loss of fuel economy. Some HSR45s are

fitted with #35 pilot jets and these may be too rich for well-tuned engines. See the tuning

manual elsewhere on this website.


58/61


Poor MileageJet needle:

There are four different jet needles. Their part numbers are: J8-8DDY01-95, -96, -97 and 98

(J8-8CFY02-xx for the 45 & 48). We commonly refer to them as a dash 97, dash 98, etc. The

current standard jet needle is a -97.

The only difference between each needle is the diameter of the straight part of the needle.

This is the portion of the needle that controls air/fuel mixture strengths between idle and

approximately throttle. So, when a mixture change needs to be made in this range, it isnecessary to exchange jet needles. Raising or lowering the needle has no effect on mixtures

below throttle.

The jet needle is both adjustable and replacable. Its height can be adjusted (via an E clip and

five grooves) to change mixture strength between and 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.


59/61


Poor MileageSpeed:

Fuel consumption increases dramatically with speed. For instance: If you wish to double the

speed, your engine must produce approximately eight times as much power. Thus, if 20

horsepower gets you 100 miles per hour, you'll need 160 HP to go 200. From this

relationship it is easy to understand why fuel economy drops so dramatically between

normal cruising 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 be better 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 given speed than the lighter

machine.


60/61


Poor Mileage

An engine that has been modified to perform best in a higher-than-normal rpm range may

suffer a dramatic loss of fuel economy if 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.


61/61


Manifolds

7: Which Manifold?

The great majority of HSR installations use the stock intake manifold that has been fitted to all

Big Twins and Sportsters for more than 20 years. This manifold is an excellent performer. It is

reliable, has excellent airflow and is available. We recommend its use.

However, Should you choose to fit a Mikuni HSR45 or 48, you must fit a manifold other than

the stock one. Also, if your engine has larger ports (the large S&S motors for example) or the

heads have a non-standard spacing (many S&S strokers or some other clone engine designs),then you must fit a non-standard manifold.

Mikuni produces an alternate manifold design. Our manifold's two piece construction allows us

fit different rubber flanges that accept 42, 45 or 48 millimeter HSR carburetors.

All current (May 2002) Mikuni manifolds are machined to fit stock (40mm) intake ports and

cannot be used with large (usually 45mm) ports. Also, they are machined to fit engines with

stock cylinder head spacing only. We make both Evo and Twin Cam versions of our manifold.

Crazy Horse Carb School

Documents

Transcript of Crazy Horse Carb School