HSR Carburetor
Total Kits
Installation & Tuning Manual
HSR42 Kits:
’84 - ’99 Evo Big Twin
’99 - present Twin Cam
’84 - ’99 Evo Big Twin
’84 - ’99 Evo Big Twin
’99 - present Twin Cam
HSR45 Kits:
Revised 5/10/00
Total Kit Installation
The HSR series carburetors are precise yet durable instruments; however, like any other piece of fine equipment, they
require correct installation and reasonable care to assure optimum performance and long life. Extra time spent during
installation will pay off in both short and long term performance and reliability.
This Mikuni HSR carburetor kit is designed to be a bolt-on application, and as such, is set-up and jetted properly for
many applications. However, since many Harley-Davidson motors are highly modified, re-tuning may sometimes be
necessary. We include the Tuning Section for that purpose.
NOTE: Carburetor Kits not designated as C.A.R.B. exempt, are not legal for motor vehicles operated on public highways in
the state of California, or in any other states and countries where similar laws apply.
Application: This installation manual applies to 1340 Evo Big Twin and Twin Cam models
Required Tools:
These are the tools typically required to remove the stock carburetor and install the HSR carburetor kit.
1. Allen wrenches, 1/8”, 5/32”, 1/4” & 5/16”
5. Slotted and #2 Phillips screwdriver
2. Standard wrenches, 3/8”, 1/2”, 9/16”, 11/16”, 3/4”
6. Blue Loctite or equivalent
3. Small adjustable wrench
7. T25 Torx (’96–present)
4. Diagonal cutters (dykes)
8. Shop manual (for your model).
Notes, Cautions and Warnings
Statements in this manual preceded by these words are
very important:
Gives helpful information that can make a job easier.
Indicates a possibility of damage to vehicle if instructions are
not followed.
Indicates a possibility of personal injury or vehicle damage if
instructions are not followed.
Read these instructions carefully before you begin
installation of your HSR kit. All procedures should be
followed exactly as described in this manual, paying
particular attention to the following:
A moderate level of mechanical skill is required to install this
carburetor kit. After reading these instructions, if you have any
doubts, we recommend that you have a professional install it
for you. If you install the kit yourself, we recommend that you
also use the applicable shop manual for your motorcycle.
1. Mikuni HSR series carburetors require the use of a push/pull
throttle assembly to assure positive closing of the throttle
valve. The high vacuum encountered, as well as dirt ingested
into the carburetor when running without an air filter may
hinder the throttle valve from closing promptly.
2. The throttle cables should be routed freely (without sharp
bends) between the throttle twist grip and the carburetor.
The throttle cables should not be pinched by the installed
fuel tank, nor should they be pinched, pulled or restricted by
the motorcycle bodywork and/or the fork assembly when it is
turned through the full range of motion.
3. Gasoline is extremely flammable and is explosive under
certain conditions. Before attempting to install or service
your Mikuni carburetor, follow these fire safety procedures:
a. Make sure your work area is well ventilated and
free from any source of flame or sparks, i.e. ,
appliances with pilot lights, such as water heaters,
clothes dryers, space heaters, etc.
b. Never look directly into the bore of the carburetor
while the engine is running as injury may result
from possible backfire.
The Mikuni intake manifold, included in these kits, requires the
flanges and seals from a ’90 and later “Big Twin.” If you do not
have these parts, you will need to to get them from a HarleyDavidson dealer. The required part numbers are:
1. 27009-86A Flange 1 ea.
2. 27010-86A Flange 1 ea.
3. 26995-86C Seal
2 ea.
1. Disconnect the battery ground wire from the battery.
2. Turn the fuel petcock to the “OFF” position.
3. Elevate the rear of the fuel tank for better access to
the carburetor area.
4. Remove the coil, horn or other devices on the left
hand side of the engine to allow free access to the
intake manifold bolts.
5. Remove the complete air cleaner assembly, including the backing plate.
6. Disconnect any vacuum hoses and the fuel hose
from the stock carburetor.
7. Disconnect the choke cable from the frame.
8. Remove the stock carburetor.
9. Evo: remove the stock manifold.
10. Twin Cam: unplug MAP sensor, remove manifold and
then remove the MAP sensor from the manifold.
Intake Manifold:
Twin Cam only: Install MAP sensor in Mikuni manifold.
1. Attach the Mikuni manifold to the engine. Align the
manifold before tightening the bolts. The flange
surface of the Mikuni manifold should be parallel with
the front of the engine’s cam cover. Use a level or
angle meter to check this. Tighten the manifold bolts.
2. Attach the rubber flange to the Mikuni manifold with
the 5/16”x3/4” bolts and washers.
To ease installation, use liqiud detergent to lubricate the
manifold seals.
We recommend that you use the Mikuni choke cable that
is furnished with the carburetor. However, you can use
the stock Harley cable (’90–’99 models only) if you follow
the Twin Cam choke cable instructions.
Twin Cam:
1. Remove the Harley choke cable from the stock carb.
2. Remove the Harley spring and plunger from the
Harley choke cable.
3. Remove the Mikuni spring and plunger from the
Mikuni carburetor.
5. Install the Mikuni spring and plunger onto the Harley
choke cable. Change nothing else; be sure to use
the Harley plastic nut, not the Mikuni nut.
6. Install the new assembly into the Mikuni. Be careful
to only gently tighten the plastic nut.
Figure 1: Harley nut with Mikuni spring & plunger
Figure 2: Choke cable adjustments
An optional choke cable mounting bracket is included in the
Evo kits for custom installations.
Throttle Cables:
Note: The HSR carburetor uses the same cables that fit
the stock CV (constant velocity) carburetor HarleyDavidson has been using since 1990.
However, if your Harley is fitted with some other carburetor, you may need to purchase a set of cables designed
for the late CV-type stock carburetor. Be sure to get cables
made for your bike’s model year.
1. Route the throttle cables with large radius curves and
so that they do not interfere with other components.
2. Screw the cable adjusters together to make them as
short as possible.
3. Connect the “close” cable first (see Figure 3).
4. Install the “open” cable next (see Figure 3).
5. Adjust the opening cable until the slide can be opened
fully. Snug the adjuster lock nut.
6. Turn the handlebar to the right and adjust the throttle
free-play with the closing cable adjuster to approximately 1/8” (see Figure 4).
1. The control cables must not pull tight when the handlebar is
turned fully left and right. Also, be sure that the cables clear
the fork stops at the steering head so they are not pinched
when the handlebar is turned.
2. It is important to check for adequate clearance between the
cable elbows and the gas tank when the handlebar is turned
from lock to lock.
There must be some free play at all handlebar positions to
avoid binding in the throttle cables.
Air Cleaner and Breather:
Use the supplied brackets, breather kit parts, O-rings,
bolts & washers as required. See illustrations below.
Evo Engines:
’84-’91: Use 2 ea. 5/16”x3/4” bolts, washers and O-rings.
’92: Use 2 ea. ½”x ¾” bolts and flat washers.
’93-’99: Add the included breather assembly (KHS-020).
TwinCam: Use 5/16” flat washers and nuts together with
the KHS-030 breather kit (KHS-030).
Figure 3: Cable installation guide
1. Loosely attach the brackets and/or breather parts to
the cylinder heads with the appropriate hardware.
2. Remove the rubber stud protectors from the Mikuni air
filter and place the two backing plates on the studs.
Position the air filter up to the carburetor while aligning
the brackets with the filter studs. Once the filter is in
place, tighten the filter clamp. Then, secure the filter to
the brackets with the enclosed 5/16” nuts and flat
washers (see Figure 9).
3. Cut out the thin membranes in the back of the filter and
insert the plastic elbows. Use the 5/16” i.d. hose to
connect the elbows to the breather fittings.
4. Use the enclosed cover screw to attach the Mikuni air
cleaner cover to the air filter.
Hose Routing:
Figure 4: Throttle free-play
Cable Lube:
Remove upper throttle housing and inject half the cable
lube in each cable.
1. Use the small hose clamps to secure the fuel hose to
the petcock and the carburetor. The screw clamp goes
on the petcock end and the spring clamp attaches to
the carburetor end.
2. Evo: Attach the V.O.E.S. (Vacuum Operated Electronic
Switch) hose to the carburetor.
NOTE: If required, a 9” piece of vacuum hose is
included in this kit
3. Twin Cam: Attach the petcock vacuum hose to the
(see illustration in tuning section of this manual for the
location of the fitting on the carb.).
4. Route the overflow hose from the bottom of the
float bowl to below the engine. Do not connect this
hose to any other hoses.
If you are not using the V.O.E.S., be sure that you cap off the
vacuum fitting on the carburetor (refer to page TM8).
You may have some hoses remaining after your installation.
Since this is a performance application only, some hoses and
related hardware may no longer be required.
Figure 5: Cable lube procedure
Cable lubrication is important. Dry cables can result in sticky
throttle action, slow return to idle and excess cable wear.
Re-connect the battery and re-assemble the remainder
of the motorcycle. Turn the petcock on and start the engine
as you normally would. After it is warm, set the idle to the
recommended 1,000 to 1,100 rpm. To learn more about
adjusting your HSR carburetor, refer to the “Tuning the
HSR” section of this manual.
Inspect and, if necessary, clean the air filter every 5,000
miles. The K&N brand filter supplied with this kit is very
effective. It filters extremely small particles out yet does not
substantially interfere with air flow into the engine. When you
clean the filter, be sure to follow the cleaning instructions
included with this kit.
Figure 8: ’93 – ’99 Evo breather (KHS-020)
Figure 6: Air cleaner assembly guide
Figure 9: Twin Cam breather (KHS-030)
Figure 7: ’84 – ’92 breather installation
Kit Contents:
Twin Cam: 42-19, 45-4
Evo: 42-8, 45-2
42-19, 45-4
Air Filter, 2.5”
Air Filter, 3”
Cover A/C (Chrome)
1 1
1 1
Rubber Flange
Rubber Flange
Breather Kit
Main Jet
Main Jet
Main Jet
Main Jet
Nut, 5/16”
2 2
Plate, Back
2 2
Hose Clamp
1 1
Flat Washer, 5/16”
2 2
Hose Clamp
1 1
Hose, Fuel
1 1
Cable Tie
4 4
Cable Lube
1 1
Note: The 45-3 Kit does not include a manifold. Otherwise, it is identical to the 45-2 Kit.
42-8, 45-2
Air Filter, 2.5”
Air Filter, 3”
Bracket, Air Filter
2 2
Cover A/C
1 1
4 4
Bolt, ½”-13 x 1”
2 2
½” Washer
2 2
1 1
Rubber Flange
Rubber Flange
Breather Kit
Main Jet
Main Jet
Main Jet
Main Jet
Hose Clamp
1 1
Choke Bracket
1 1
Bolt, 5/16” x ¾”
4 4
Nut, 5/16”
2 2
Hose, 3/16” x 9”
1 1
Elbow, Plastic, 3/8”
2 2
Plate, Back
2 2
Screw, Cover
1 1
Hose Clamp
1 1
Flat Washer, 5/16”
6 6
Hose Clamp
Hose, Fuel
Cable Tie, 7-1/2”
Choke Cble Assy
Cable Lube
Tuning Manual
Revised 5/10/00
Tu n i n g the HSR
Your Mikuni HSR is fitted with the tuning parts we
found to work with the great majority of engine performance modifications. However, the large number of
differing exhaust systems and cams available makes it
impossible to accommodate all possible combinations
with one carburetor set-up. Your HSR will almost certainly run correctly on your engine without exchanging
any parts. But, if it doesn’t, you may alter its tuning to
suit your engine’s needs by following this guide.
Some exhaust system designs strongly interfere with
carburetor tuning. For instance, it is very difficult to get
smooth and responsive carburetion through the entire
rpm range with straight pipes and completely open
exhausts. In addition, very small volume, small diameter
mufflers are often ‘seen’ by the engine as straight pipes
and present the same tuning difficulties.
Very long duration cams often cause relatively poor
running below about 3,000 rpm, depending upon the
individual cam’s intake valve closing point. Such cams
cause reverse airflow out the mouth of the carburetor
(often called: “reversion” or “stand-off”) that can be
mistaken for a carburetor tuning problem.
Harley-Davidson Screamin’ Eagle performance parts
are proven and predictable. If you have any doubts about
a particular exhaust system, air cleaner or ignition, you
may substitute the standard Harley Screamin’ Eagle
parts as a “reality check.”
When re-tuning is required, it usually involves small
alterations to the idle and/or main system. The following
paragraphs supply enough detail information to make
such changes an easy matter for an experienced
Finally, please note that there is simply no point in
attempting to tune any carburetor unless the engine is
sound and in a good state of tune. If you have any
doubts about the general condition of your engine, have
it checked by your dealer or an experienced mechanic
before attempting to fine-tune your new Mikuni carburetor.
We have found that a certain percentage of HarleyDavidson Evo and Twin Cam engines have minor air
leaks between the manifold and heads. The leaks affect
air/fuel mixtures at low throttle settings and can be
troublesome at idle. For best performance, it is important
that you test for such leaks and eliminate them if any are
It is easy to test for intake manifold air leaks: With the
engine warm and idling, spray WD-40 around the
junctions of the manifold, carb and heads. If the engine
changes from its steady idle, if it surges, misses, etc
then there is an air leak that must be corrected if the
engine is to run its best.
Because of the frequency of air leaks, we recommend
that you replace all three manifold seals when you install
your new Mikuni carburetor. These parts are available from
any Harley dealer.
If you lubricate the seals just before installation, they
will seat more smoothly and are less likely to develop
leaks. Liquid dish detergent works well. WD40 is also a
good choice but evaporates quickly.
It is important that the manifold be aligned before the
lubricant sets or evaporates. Otherwise, the parts may
stick to the seals, which, in turn, can cause leaks when the
parts are moved into alignment.
For the quickest and most accurate results when fine-tuning
your HSR, we recommend using “witness” marks on the throttle
grip and throttle housing. Use masking tape on the grip and an
indicator mark on the throttle housing. Mark the tape in ¼
throttle increments from idle to full throttle. You can then identify
the throttle opening where a tuning change may be required and
then adjust the correct tuning components.
Figure: T1
The HSR42 carburetor is divided into four interdependent systems:
1. The choke system
2. The idle system
3. The main system
4. The accelerator pump system
Each of these systems has its major effects in a
different throttle range. While there may be some overlap,
each system can generally be treated as though it is
completely responsible for its range of throttle settings.
Three of the systems have replaceable components that
allow fine-tuning should the need arise.
The choke system’s purpose is to provide the rich air/
fuel mixture an engine needs to start and run reliably
when it is cold. There are no replaceable tuning parts in
the HSR42/45 choke mechanism.
The choke is designed to work correctly with the
throttle closed. Opening the throttle stops the action of
the choke.
Make sure that the stock Harley-Davidson choke cable
is fully seated in the metal elbow at the carburetor end of
the cable assembly (see installation instructions). Harley
‘s cable is stiff and can fail to fully seat in the elbow. This
condition results in poor mileage and a poor idle.
The Mikuni choke cable is more flexible and less likely
to jamb. Still, it is best to check to be sure the cable is
installed correctly.
TUNING THE IDLE SYSTEM: (idle – 1/8 throttle)
The HSR42/45 idle system has two tunable components: the air screw and the pilot jet. The air screw’s
purpose is to fine-tune the idle mixture. The pilot jet
controls the total amount of fuel passing through the idle
system. It can be changed to make larger adjustments to
the idle mixture strength.
The air mixture screw is set at two turns out from the
factory. This is the position we have found to be right
most of the time. If the screw position has been altered,
gently bottom it and re-open it two full turns.
Next, ride the bike until its engine reaches normal
operating temperature. This may require several miles at
highway speeds.
With the motorcycle vertical and the engine idling
near 1,000 rpm, adjust the air mixture screw in slowly
until the idle either slows or becomes irregular, then
begin turning the screw out until the engine again slows
or again begins to idle irregularly. Count the number of
turns between the two positions. Set the air screw midway between these too-rich and too-lean positions.
Use the Idle Adjuster Screw to re-set the idle speed.
If the engine starts to get too hot during the adjustment procedure, the resulting idle mixture will probably
be on the lean side of correct. If you have a large fan,
use it while adjusting the mixture. If you do not have one,
you may need to take time out for a short ride to cool the
engine back to normal temperature.
If the best idle is achieved with the air screw less than
one turn out, the pilot jet is too small and should be
exchanged for a larger one.
If the engine speed does not slow after two-and-a-half
turns out, the pilot jet is too large and should be exchanged for a smaller one.
Figure: T2
JET NEEDLE (1/8 – 1/4 throttle)
The straight diameter portion of the jet needle controls
the mixture from just above idle to approximately ¼
throttle. If the mixture is too rich or too lean in this throttle
range, the needle will need to be exchanged for a larger or
smaller one. HSR jet needles are available in four sizes.
Only the diameter of the straight part of the four jet
needles differ. The richest is the “-95” and “-98” is leanest.
If the needle is too lean (large in diameter), part throttle
acceleration will be flat. There may also be some detonation during part throttle acceleration from low rpm, (although this may have other causes). A lean needle also
results in an abnormally slow warm-up.
If any of these conditions exist, install a one-size smaller
needle and compare the performance
While a black, sooty spark plug is a sure sign of richness; there are more subtle signs. If your engine responds
crisply at low throttle openings when it is cold, chances are
the needle is one size smaller than it needs to be. This
assumes, of course, that the idle mixture is correctly
Poor fuel mileage is another sign of an over-rich condition.
Because of the way most of us ride our Harleys, that
richness is usually the result of a jet needle that is too
small (rich). The color of the end of the exhausts is a sign
of mixture strength. Dark gray with some black is normal
for today’s lead-free gasolines.
JET NEEDLE (1/4 - 3/4 throttle)
The tapered portion of the jet needle begins to rise out
of the needle jet at about ¼ throttle. From that point until
the main jet takes over near ¾ throttle, the jet needle taper
is the main influence on mixture strength.
When testing, consider the rpm effects of any accessory cam you may have installed. Long duration cams tend
to perform poorly below some critical minimum rpm. If you
attempt to test below this rpm, the engine may seem soft,
flat and unresponsive. No carburetor can compensate for
the engine being “off the cam.”
All jet needle and main jet testing should be done with
the engine near the middle of its rpm range, but high
enough to be “on the cam.” With the engine at operating
temperature, accelerate at ½ throttle in third or fourth gear.
If acceleration seems soft or flat and the engine is slow
to respond when the throttle is quickly opened from the ½
to the ¾ position, the mixture is too lean. Raise the needle
one notch and repeat the test (refer to page: TM6).
If acceleration is crisp but the engine hesitates or
staggers as the throttle is quickly shut down from ¾ to ½
throttle, the mixture is too rich. Lower the needle one notch
and repeat the test. The needle will be correct when
acceleration is crisp at mid-rpm yet the engine does not
load up during throttle shut down.
See the “How To” section at the back of this manual for jet
needle adjustment and removal procedures.
Please do not disassemble the throttle lever linkage. It is
not normally necessary to remove the bolt to change position
of the jet needle. However, if you do remove the throttle shaft
bolt, be certain that you:
1. Apply a drop of blue LoctiteTM to the threads.
2. Torque the bolt to 18in./lb.
3. Replace the tab washer and bend it up until it is flush
against one of the bolt head flats.
MAIN JET: ¾ — full throttle
The main jet controls fuel flow from ¾ to full throttle.
It is the last you need tune and is the easiest to get
An accurate method for choosing the correct main
jet is to accelerate at full throttle between two points and
note the speed at the second point.
1. Select markers that are far enough apart so the
engine gains about 2,000 rpm (in third or fourth
gear) between the two.
2. When you pass the first marker, quickly roll the
throttle fully open and note the speed as you pass
the second marker.
3. The jet that gives the highest speed is the correct
one. This method is simple but effective.
1. The best method is to use a dynamometer. The
main jet than produces the most power is the
correct one. If two jets deliver the same power, use
the smaller one.
The accelerator pump has two adjustments and
one replaceable tuning part, the accelerator pump
nozzle, that can be adjusted or changed to refine the
performance of the accelerator pump system.
The accelerator pump injects a metered and adjustable amount of fuel into the engine when the throttle is
opened from or near its closed position.
The beginning point of the pump stroke is adjusted
with screw #1 on the white plastic pump lever. To start
the pump sooner (smaller throttle opening), back the
screw out. To start it later, turn the screw in.
The end of the accelerator pump stroke is adjusted
by screw #2 located on top of the carburetor just behind
the pump lever. Best performance is generally achieved
when the pump stroke ends before 2/3 throttle.
The accelerator pump nozzle size (#50, #60 or #70)
determines the rate at which fuel is delivered to the
throat of the carburetor. A larger nozzle delivers fuel at a
higher rate than a smaller nozzle.
Figure: T3
4. If a jet or passage does become plugged, use only
carburetor cleaner and compressed air. DO NOT push
a drill or any other object through the jet or passage to
clean them.
1. Invert the carburetor and remove the float bowl.
2. The float assembly’s actuator tab should just contact
the Needle Valve assembly when the bottom of the
float is 18mm from the carburetor bottom surface as
3. Bend the actuator tab to adjust float level.
Figure T4: Accelerator Pump Adjusters
The standard nozzle size is #70. If the engine seems
to run too rich when the throttle is first opened the
nozzle may be too large and the fuel delivery rate too
high. In this case, fit a smaller nozzle.
The #1 screw is normally adjusted to establish
a gap of about 2mm (about the thickness of a nickel)
between the white plastic lever and pump rod end. If
there is a hesitation just off idle, reduce the gap.
The #2 screw is normally adjusted to stop the pump
action at about 2/3 throttle. If you tend to accelerate from
low speeds in a tall gear, you may need to adjust the
pump for a longer stroke. If you tend to downshift before
accelerating from low speeds and/or have a lighter bike
then you might consider shortening the pump stroke.
Figure T5: Float Level
For best results, the accelerator pump nozzle should be
pointed directly at the jet needle. The nozzle is held in place
by the friction of an O-ring and can be turned easily with a
pair of long nosed pliers. Nozzle adjustment should be made
with a minimum of pump strokes to avoid flooding the engine.
There are few moving parts in the HSR series
carburetors and they do not require frequent servicing.
However, here are a few suggestions that, if followed,
will assure good performance season after season.
1. If the motorcycle is to be stored for any length of
time, the carburetor float bowl should be drained.
2. The float bowl drain plug (main jet access) should be
removed periodically and cleaned of any sediment
that may have accumulated during long periods of
3. DO NOT drill or modify any part of this carburetor for
any reason, as the result will surely be more problems.
Check the condition of the float bowl O-ring. If it is worn or
does not not fit in the groove correctly, replace it.
Mikuni, Screamin’ Eagle and many others are made by
K&N and do not need frequent cleaning. A cleaning
interval of once a year or 5,000 miles is often enough.
However, if you ride in very dusty conditions, clean the
filter when it is obviously dirty over 50% or more of its
1. Tap the element to dislodge embedded dirt; then gently
brush with a soft bristle brush.
2. Roll the filter element in a large shallow pan of K&N air
filter cleaner (Simple Green, 409, etc) with the depth
set to ¼ of a pleat. Remove immediately and let sit for
approximately 10 minutes.
IMPORTANT: Do not use
gasoline or cleaning solvent to wash the filter element,
as this will damage the filter.
3. Rinse the element with low-pressure water. Always
flush from the inside of the filter out to ensure that
dirt is washed out of the filter and not into it. IMPORTANT: Air dry only; do not use compressed air.
4. After the filter is completely dry, re-oil sparingly.
Apply K&N filter oil with one pass per pleat. Wait 10
minutes and re-oil any white spots.
1. Back out the idle speed adjuster five or six turns
to ensure that the throttle valve is fully bottomed.
2. Remove the three top cover screws and remove
the cover.
3. Loosen the Allen screw located on the slide
(long 2.5mm Allen wrench).
4. Swing the clip aside.
5. With the slide bottomed, remove the jet needle.
You may use a long-nose pliers or tweezers for
this. Be sure to save the small plastic washer
under the jet needle clip.
6. To reassemble, reverse steps 1. through 5. Be
sure to snug the Allen screw and replace the
plastic washer under the jet needle clip.
1. Remove the jet needle per previous instructions.
2. Place the open end of the jet needle clip against a
hard surface.
3. Cover the clip area with your hand (to prevent loss of
the clip) and press the needle down to snap the clip
off the needle.
4. Place the clip in the desired groove on the needle.
5. Place the clip against the hard surface with the open
end up.
6. Again cover the clip area with your hand and press
down on the needle to snap the clip into place.
1. Turn the fuel petcock to the OFF position.
2. Use an 11/16” or 17mm wrench to loosen & remove
the drain bolt.
3. Use a good fitting slot type screwdriver to remove
the main jet.
4. Reverse steps 1 through 3 to replace the main jet.
The jet need only be hand-tight; do not over tighten.
1. Turn the fuel petcock to the off position.
2. Remove the four screws retaining the float bowl (#2
Phillips). Remove the float bowl.
3. Use a good fitting slot type screwdriver to remove
the pilot jet.
4. To reinstall the pilot jet, Reverse steps 1 through 3.
Start the jet by hand, as the threads are small and
relatively fragile. Do not over tighten; snug is enough.
1. Turn the fuel petcock to the OFF position.
2. Remove the four screws retaining the float bowl (#2
Phillips). Remove the float bowl.
3. Use your finger to push the nozzle out of the throat of
the carburetor (see exploded view).
4. Push in new nozzle/O-ring assembly. Aim the nozzle
toward the jet needle.
5. Re-install the float bowl.
6. Turn the fuel petcock on and work the throttle several
times to prime the accelerator pump.
7. Use long-nose pliers to turn the nozzle until the fuel
stream points to the middle of the carburetor throat.
OVERFLOW: Fuel runs out of carburetor through
overflow tube on bottom of float bowl, from vent
fitting on side of carburetor or from body/float bowl
Foreign matter around fuel needle valve & seat:
Includes paint flakes, rust or bits of fuel hose.
Stock or clogged tank vent:
Factory one-way tank vents can cause pressure in
tank. Clogged tank vent can result in fuel starvation.
Deteriorated gasoline causing sticking fuel needle valve.
Loose float bowl or damaged float bowl packing.
mileage. Highly tuned engines often require leaner jet
needles as do motorcycles operating at high altitude.
Stock or clogged tank vent:
Factory one-way tank vents can cause pressure in
tank. Clogged tank vent can result in fuel starvation.
Air leak:
It is common that Evo series engines have intake
manifold air leaks. An air leak can lead to a lean
running condition and even detonation at part throttle.
Loose jets:
A loose main jet can result in an overly rich mixture.
POOR FUEL ECONOMY (at normal cruise speeds):
Mikuni installations normally deliver fuel economy
very close to that of the completely stock Harley.
Choke cable routing:
The choke cable must be fully bottomed in the metal
elbow (at carburetor end). There must be some
perceptible free-play in the choke knob.
Stock or clogged tank vent:
Many current stock Harley tank vents do not allow
gasses to leave the tank and pressure may develop
from engine heat. Open the vent to allow two-way air
flow. Clogged tank vent can result in fuel starvation if
the vent valve becomes stuck or partially blocked.
Fuel/Air mixtures:
Incorrect jet needle or pilot jet for tuning set-up or
altitude. The speed range in which mileage is usually
recorded is controlled by the straight portion of the
jet needle and pilot jet. The needle taper and main
jet size seldom have an effect at normal cruising
Loose jets:
A loose pilot or main jet can affect fuel mileage.
flat acceleration, misfiring.
Clogged tank vent:
Factory one-way tank vents (located in the tank cap on
most models) sometimes fail to allow enough air into
the tank to replace fuel flow at high throttle openings.
The result is a partial vacuum and fuel starvation.
Fuel flow restriction:
Clogged fuel filter or fuel petcock screen (located inside
tank) can result in fuel starvation.
Incorrect main jet size:
A too large or too small main jet can lead to poor power
Clogged main jet:
A partially clogged main jet may allow enough fuel flow
for part-throttle operation and yet starve the engine at
full throttle.
Dirty air filter:
Stock air filters can become dirty enough to restrict air
flow and power output.
Irregular idle speed, too rich or too lean.
It is common for Evo series engines to have intake
manifold air leaks. Air leaks result in an irregular idle
that does not respond to mixture screw adjustment.
Idle mixture adjustment:
The pilot air adjusting screw is not adjusted correctly
resulting in a too rich or too lean idle mixture setting
(see p. TM3).
Incorrect pilot jet:
A jet that is too large or too small.
Loose jets:
A loose pilot or main jet can affect idle performance.
Overly rich or lean at normal cruise speeds, detonation at part-throttle.
Choke cable routing: (see p. TM3)
Incorrect jet needle:
A too-rich or too-lean jet needle for engine tuning
set-up or for operating altitude can cause poor
When the engine is cold only (choke off):
This can be considered normal. A cold engine does not
vaporize fuel completely which can result in an overlylean mixture and backfiring through the carburetor.
Long duration cams:
Cams with late intake valve closing points encourage
backfiring at low rpm.
Exhaust systems:
Some very open or incorrectly designed exhaust
systems encourage backfiring. Stock length and
diameter header pipes together with typical small slipons, like those furnished by Harley-Davidson, typically
do not have this problem.
Incorrect accelerator pump adjustment:
If the accelerator pump adjustment is set to start too
late, backfiring may occur due to an overly lean mixture
just off idle.
closed, especially noticeable from high rpm, is not
caused by lean mixtures, although lean mixtures can
contribute to its intensity. This is a normal condition
when using high performance exhaust systems.
Open exhaust system:
High performance mufflers with large exit area or lowrestriction baffles.
Air leak in exhaust system: Air entering at the junction
of the header pipes and mufflers can cause popping
upon decceleration.
Ignition failure: Out-of-time ignition together with misfiring may lead to loud backfiring. Such backfiring
usually occurs irregularly and at large throttle
HSR Accessories
Optional Tuning Components and
Accessories for HSR Carburetors
These parts may be ordered through your local Mikuni dealer
Accelerator Pump Nozzles
Std. for Sportster kits
Std. for Big Twins
Mikuni Jet Kit
Jet Tuning Service Kit for HSR42/45 . . . ……….. KHS–025
Jet Kit Contains: 18–Main Jets (2 each 150 thru 170)
10–Pilot Jets (2 each 22.5 thru 35)
3–Needles (1 each –95, –96, –97)
2–#60 Pump Nozzle
4–Needle E-Ring Clips & Washers
1–Plastic Box
Low & High Speed Jets:
Pilot (Idle to ¼), increments of 2.5
(Standard: 25, normal range: 20 to 35)
HSR42/45 Carburetor Rebuild Kit
N100.604 – (Size)
Carb Rebuild Kit . . . . . . . . . . . . . . ………….....….KHS–016
Main (¾ to Full Throttle), increments of 2.5
(Standard: 160, normal range: 150 to 180
See exploded view drawing for kit contents.
Short Idle Screw
Idle Adjuster Screw (Short) . . . …………….... 990-605-065
Jet Needles (Mid-Range: 1/8 to 3/4)
J8-8DDY01 – 95
J8-8DDY01 – 96
J8-8DDY01 – 97
J8-8DDY01 – 98
Standard HSR42 Jet Needle straight diameter is 2.97 mm.
This portion of the needle affects tuning from idle to approximately ¼ throttle opening.
(See item #59a in exploded view of carb.)
Do not modify idle screw or any of its component pieces. If the
screw is removed, be sure it is re-installed with the components
in place as illustrated in exploded view.
Mikuni American Corporation
8910 Mikuni Avenue
Northridge, CA
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