Harley Davidson Carburetor

Harley Davidson Carburetor
Harley Davidson Carburetor
Performance Tuning
Tuning Harley Davidson carburetor is simpler than most think and can be performed with
a few common tools. This simple procedure is a great Harley tech tip that applies to all
Harley carbs from 1989 to present that use the CV style Harley Davidson carburetor..
Preparing the carburetor for tuning will require removal from your Harley-Davidson's
engine but this is easily accomplished. Begin by shutting off the petcock fuel valve and
starting the engine to allow all fuel within the carburetor bowl to be emptied. Remove the
aircleaner assembly including the backing plate which is attached to each head with a
banjo bolt.
This is a good time to inspect these bolts for obstructions in their passages. Remove the
choke cable from it's mounting bracket on the opposite side of the bike.
Note: Before proceeding to rejet your carburetor it is recommended that you perform
the following tuning procedure. Once the the mixture has been properly tuned there is
often no need to rejet the carb.
The choke cable will stay attached to the carburetor during this procedure. Disconnect
the fuel line from the fuel inlet on the carburetor or the opposite end connected to the
fuel petcock, whichever is simpler to access. The hose is likely fastened using a special
crimped clamp. This may be cut or pried off to remove since you won't be using it again.
Be sure to have a new hose clamp available.
Next loosen both throttle cables from their adjusters located just beyond where they exit
the throttle grip. A couple turns is usually all it takes to give you enough slack. If you
count how many turns each adjuster is loosened then you can return them to the exact
adjustment when reinstalling your carb. This is a good time to label each cable to avoid
any confusion when reconnecting them to the carb's throttle cam. A simple "Top" and
"Bottom" should suffice when tagging each cable.
Now that you have enough slack in the cables you can pull the carburetor away from the
manifold. A Harley carb is only held to the manifold with a slip fit rubber boot. Gently
rock or twist the carb back and forth as you pull it away from the engine. Remove the
cables that you tagged and remove any vacuum hoses. If your model has multiple
vacuum hoses it would be a good idea to label these as well.
With the carb removed, place upside down on a sturdy work surface. Do not remove the
bowl at this point to prevent debris from entering the carburetor.
The CV style Harley carburetor has a small cylindrical tower
protruding from the bottom rear of the spigot (behind the
bowl). The tower is plugged with a soft metal insert covering
the mixture screw. Gaining access to this screw is key to fine
tuning and must be remove. The metal plug is very soft and
only requires a household drill and 7/64" to 1/8" bit.
Secure the carb in either a vise or by other means that will
allow the carburetor to remain steady. Drill a hole into the
plug making sure not to "punch" through too fast. You don't
want to damaged the mixture screw just below the plug.
Allow the drill to slowly cut into the plug rather than push.
Tip: To keep from drilling too far into the plug, it has been suggested to wind electrical
tape around your drill bit quite a few times about 3/16" from the tip. This will create a
stop to keep the bit from drilling too deep.
Pry the plug out using a pick or awl. You can also thread a sheet metal screw into the
drilled hole and use this to pull the plug out. Now that the plug is removed clean the area
around the mixture screw so no metal fragments remain. At this point there are two
methods for adjusting the mixture.
Adjustment Method: Using a small flat head screwdriver turn the screw clockwise until
Count how many turns it takes to reach the closed position. Mark the screwdriver if
needed to properly count each turn. Now turn the screw out counter clockwise stopping
at a 1/4 turn beyond that which you originally counted. For example, it you turned the
screw in 1-1/2 turns then unscrew it 1-3/4 turns. This is your base starting point and
alone will allow your idle mixture to be slightly richer than the factory's EPA setting. In
many cases this will be the ideal setting.
As an alternative to adjusting the mixture screw with a screwdriver, many prefer
using an EZ-Just mixture screw to ease adjustments and fine tuning.
EZ-Just mixture screw (HD aftermarket)
If the mixture screw has been reset by the dealer or previous owner (evident by the plug
already being removed), turn the screw clockwise until it seats. Now turn the screw
outward 2 turns to establish a starting point. The same procedure applies if using an EZJust screw.
Reinstall the carburetor back on your bike by reversing the steps taken during removal.
Be sure to replace the fuel hose clamp and vacuum lines. It may be a matter of dexterity
but I prefer to install the cables before pushing the carb back onto the manifold. Make
certain the carb firmly seats back onto the manifold boot. Test the throttle for binding
and smooth operation. Double check each hose and connection.
The air cleaner assembly MUST be installed prior to starting the engine, not only to hold
the carburetor in place but to prevent having the carb backfire in your face while tuning.
Start the engine as normal and bring up to operating temperature prior to fine tuning.
Let the bike idle for no more than 5 minutes. The modified carburetor should allow your
bike to run well enough for a mild test run around the block to speed up the warm-up
With the engine warmed up and at idle you may now fine tune the idle mixture screw for
optimal performance. Acquainting yourself with the adjustment screw location at the
bottom rear of the carburetor prior to running the engine is advised, which also prevents
burning your hands. You will need a small screw driver for adjusting the screw unless an
EZ-Just has been installed. With the engine idling slow (no more than 900 rpm), turn the
screw inward (clockwise) SLOWLY until the engine starts to stumble. You are working
with the screw upside down so check to make sure you are turning the screw clockwise
or inward.
Note: Be careful not to allow the screw to fall out as there is a very small spring, washer,
and o-ring that will fall out as well. These items known as the mixture screw packing kit
are not available from the dealer or manufacturer, however if you should lose these parts
there is an aftermarket replacement kit available
If the engine will not idle on its own during this procedure, adjust the idle set screw on
the throttle side of the carburetor until it idles correctly. Now turn the mixture screw
outwards (counter-clockwise) until the engine begins to run smoothly, then add 1/8 of a
turn. Maintain proper idle speed and repeat the adjustment each time you adjust the idle
speed. Blip the throttle a couple of times and observe the results. If the engine responds
quickly with a smooth blast and no backfiring through the carburetor, you have your idle
mixture right. If backfiring occurs through the carburetor then adjust the idle mixture
screw out another 1/8 turn. Normally, the mixture screw should only require 2 to 3 turns.
Anything above 4 turns indicates the pilot jet is too small.
Optimal setting on most Harley's is approximately 1/4 turn clockwise when backed out
from the point of backfiring (coughing).
Take your newly tuned bike for a ride and note how it idles and responds off idle. If you
experience any coughing through the carburetor, adjust the mixture out another 1/8 of a
Black smoke seen from the exhaust at idle or a feeling of sluggishness off idle indicates
you may have set the mixture too rich. If your bike is now idling steady and responds
well from a start then you are all set. If your engine still runs lean you should move on to
rejetting your carburetor.
The same stock Harley Davidson carburetor has been used on all production bikes from
1989 to 2006 due to it's reliability and ability to adapt to different conditions. With just
the right amount of tuning there's no reason why you can't have some of the same
performance gains advertised by the major racing carb manufacturers.
Harley Carburetor Jetting
Harley carburetor jetting should only be performed after completing the fine tuning
procedure described under Performance Tuning and your bike is still running too lean.
A good rule of thumb is to replace the Pilot Jet first and only replace the Main Jet once
the engine's idle and midrange are satisfactorily tuned. The Main Jet is only used at 3/4
to full throttle and has no effect on the idle or midrange mixture.
Main jet replacement should be reserved until after the slow idle jet is replaced and
mixture is tuned unless a lean condition is apparent during full throttle. Harley carburetor
jetting can be accomplished with minimal mechanical knowledge.
Tools to perform this task include just a simple set of
screwdrivers. You will also need an assortment of jets.
Remove the 4 screws securing the bowl to the base of the
carburetor and remove the bowl. Using a narrow 1/8" flat
head screwdriver unscrew the Pilot Jet from within the
orifice pictured. The jet size is stamped into the top of the
jet (i.e. 42). Be careful not to strip the head of the jet.
Pilot Jet location
With a flat head screwdriver unscrew the Main Jet from
the brass needle jet holder (aka Emulsion Tube).
Note the jet size stamped into the
165). There is no need to remove
unless required for cleaning. I don't
Tubes" as they change the mixture
with stock jets.
top of the jet (i.e.
the emulsion tube
recommend "Power
and ability to tune
Main Jet
Replace the Pilot Jet with one size larger. This of course
assumes that you are starting out with the stock jet size.
Harley Davidson Pilot Jets for CV Carburetors are
normally sold in sizes 40, 42, 45, 48, 50, and higher. If
your stock jet was a #42 the next size larger will be #44
or #45. Only increase the jet sizing one size at a time to
avoid an overly rich idle.
Only replace the Main Jet with one size larger after properly tuning your slow/idle jet
settings. Main Jets are sized incrementally by 5, so if your stock jet was a #175 the next
size larger will be #180. you should only increase the jet sizing one size at a time. Avoid
installing jets that are too rich as this will create a sluggish feeling at full throttle as well
as contribute to plug fouling.
Reinstall the bowl making sure to align the accelerator pump shaft and rubber boot.
Install the carburetor back onto the bike and perform the tuning procedure as described
under Carburetor Tuning.
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