CRAZY HORSE MOTORCYCLES Carburetor Class

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CRAZY HORSE MOTORCYCLESCarburetor Class
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Mikuni Motorcycle Carburetor Theory

• Motorcycle carburetors look very complex, but
  with a little theory, you can tune your bike for
  maximum performance.
• All carburetors work under the basic principle of
  atmospheric pressure. Atmospheric pressure is 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 the
  atmospheric pressure inside the engine and
  carburetor, we can change the pressure and make
  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 is
  higher outside the engine and carburetor, air
  will rush inside the carburetor and engine until
  the pressure is equalized.
• The moving air going through the carburetor will
  pick up fuel and mix with the air.

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

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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
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Drawing of Carb
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View of Carb
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Pilot Circuit
The pilot circuit has two adjustable parts. The
pilot air screw and pilot jet. The air screw can
be located either near the back side of the
carburetor or near the front of the carburetor.
If the screw is located near the back, it
regulates how much air enters 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. If
the 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.
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Slide Valve
The 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 leaner the 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 stamped into
the slide, it has a 3.0mm cutaway, while a 1 will
have a 1.0mm cutaway (which will be richer than
a 3).
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Jet Needle
The jet needle and needle jet affects carburetion
from 1/4 thru 3/4 throttle. The jet needle 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 diameter of 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.
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Jet Needle
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Main Jet
The main jet controls fuel flow from 3/4 thru
full throttle.
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 fuel flow. 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.
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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.
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Air/Fuel Mixture
The air/fuel mixture must be changes 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 short period 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
ratio is usually richer. The drawing to the
right shows the actual air/fuel ratio for any
given throttle opening.
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Working Range
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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.
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Preparing to Tune
While 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.
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Start Tuning
Clean 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.
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Altitude, Humidly, and Air Temperature
Once the jetting is set and the bike is running
good, there are many factors that will 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.
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Correction factors
Humidity 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 on and 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 the carburetor 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 the altitude
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.
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Correction factors
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Correction factors
Correction 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.
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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 size is 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 for stock 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.
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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 more accurate.NOTEThe following tuning
techniques might result in excessive (illegal)
speed and increased risk from the speed and the
necessary distraction of doing the test. We
recommend that the testing be done on a closed
course (track) or on a dynamometer, if one is
available.
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Roll Off Method
ROLL-OFFThe 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.
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Poor Mid-Range Performance
2 Poor Mid-Range PerformancePossible
Causes1.Carburetor Tuning 2.Exhaust
system 3.Too much cam 4.Ignition 5.Low
compression pressureCarburetor Tuning
Typically, mid-range performance is controlled
by the jet needle/needle jet combination. 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.
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Mid-Ranged Performance
Mikuni 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.
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Mid-Range Performance
Please refer to the tuning manual, available on
the Manuals page for instructions on diagnosing
and tuning. NoteConfusing 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 well at 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).
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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 go away, 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
pipesOpen 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.
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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
in the same rpm range as straight pipes and
exhibit similar symptoms. Long thin
mufflersLong, 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 systemsThese 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
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Mid-Range
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 that
matter, seldom deliver the best power at high rpm
either.Header pipe lengthThe 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 (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 work very well
with longer (and un-even) header pipe lengths.
Stock Harley header pipes are near-perfect in
diameter and length. Muffler sizeIt 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.
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Mid-Range
However, 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.
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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. With current
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 best work 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 rpm35   degrees 3000 rpm40  
degrees 3600 rpm45   degrees 4000 rpm50
degrees 4500 rpm 
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Mid-Range
These 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. IgnitionStock 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.
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Mid-Range
Ignitions 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.
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Mid-Range
The standard method for determining the
compression or cranking pressure of an engine 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.
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Backfire
Backfires Through Carburetor     Common
CausesIgnition 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.
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Backfire
IgnitionHarley 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 EFI touring 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.
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Backfire
Dual 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 firing stroke.When
the rear cylinder is getting a useful spark, the
front cylinder ís 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 not
produce 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.
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Backfire
Cam designThe 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 leakA 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.   
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Backfire
Carburetor jettingExcessively 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-way valve (for emission
purposes) sometimes restricts air flow into the
tank. This restriction can result in a partial
vacuum and fuel flow restriction.
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Backfire through Exhaust
Backfires in Exhaust     NoteIt 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 is closed
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
Backfire-Exhaust
Why This (normally) Happens1) 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 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
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 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 an
open exit for the popping to occur.
45
Backfire-Exhaust
Other possible causesAir LeaksAny 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 the
intensity 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
CarburetionWhile exhaust system popping may be
considered normal, it is certainly made worse by
an overly lean idle circuit.
46
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. IgnitionIf 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
Spark Knock
Detonation ("Spark Knock")Detonation, often
called pinging, is nothing less than a series of
small explosions that 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
the engine 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
Spark Knock
HOW IT HAPPENSAfter 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 and detonation 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 in a predictable way.
Peak chamber pressure occurs at about 12 degrees
after top dead center and the piston gets pushed
down the bore.
49
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 piston movement 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
Spark Knock
WHAT 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 completeTiming - 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 too soon. 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
Spark Knock
Motorcycle 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 the engine
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 the
likelihood 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
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 or less. 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 test is 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
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
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
causesChoke 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
Poor Mileage
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 cableIf you are using
the Harley choke cable (the word Choke on the
knob is white), use this procedure 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 amount of free
play before you feel the tension of the choke
return spring.
56
Poor Mileage
If there is no free playCheck 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 idle and low throttle settings.
Mikuni choke cableThe 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.  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 small
amount of free play.
57
Poor Mileage
Carburetor tuningMikuni 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 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 systemThe 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, the air/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
Poor Mileage
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.îThe 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 º throttle. So, when a mixture
change needs to be made in this range, it is
necessary to exchange jet needles. Raising or
lowering the needle has no effect on mixtures
below º throttle.    The jet needle is both
adjustable and replacable. Its height can be
adjusted (via an ìEî clip and five grooves) to
change mixture strength between º and æ throttle.
Lowering the jet needle leans the mixture and
raising it richens the mixture.Main jetThe
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
Poor Mileage
Speed 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. WeightHeavy
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
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
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 SS motors for example)
or the heads have a non-standard spacing (many
SS 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.