4
TUNING TIPS
Gasoline is the typical fuel used for
internal combustion engines. In some
racing applications methanol alcohol is
used for fuel. When using alcohol the
engine must consume approximately
twice as much fuel as with gasoline.
For this reason some parts need to be
replaced for proper tuning. The main
nozzle, main and pilot jets, and the jet
needle need to be replaced with the
proper size.
If a carburetor has been working well
and then develops a problem, the most
likely cause is dirt or contamination
in the carburetor. Dirt can plug jets or
get stuck in the inlet needle and cause
flooding. It will save time in the long
run to be sure you are working with a
clean carburetor.
If the engine performance seems to
change or idle/low speed running
seems erratic, check for loose
carburetor and manifold mounting
bolts or leaking gaskets or seals. This
can cause a vacuum leak and lean
condition.
Symptoms of a rich mixture are:
• Black smoke or popping in the
exhaust
• A “flat” running engine or dull
sound
• Runs worse as engine warms
Symptoms of a lean mixture are:
• High cylinder head or exhaust
temperature
• Backfire/popping from the
carburetor
• Runs better with partial choke
• Surging or cutting out
• Pinging or rattling sound from
engine under load
The Pilot Jet, Idle Mixture Screw and
Jet Needle will have the most effect
on acceleration from closed throttle.
A slightly rich idle mixture may help
acceleration.
Moving the position of the Jet Needle
clip is a good quick adjustment for a
change in air temp. This will help with
throttle response and corner exit.
The Main Jet will have the greatest
effect at wide open throttle but will
also affect mid-range operation. After a
Main Jet change it may be necessary to
move the Jet Needle position for best
results.
The optimum float height setting
may change from track to track. A
rough track or an engine misfire in
certain corners may require a float
adjustment. If the carburetor can be
rotated on the manifold, it may help to
turn the carburetor a few degrees to
compensate for a problem corner or for
oval tracks.
SET-UP
The throttle/pedal ratio is an item that
can be tailored to suit the driver’s style
and benefit overall engine performance.
By changing the distance from the
pedal pivot to the point the throttle
cable or linkage attaches to the pedal,
the carburetor action can be slowed
down to help throttle control on a slick
or rough track or smooth out a choppy
driver.
IT IS IMPORTANT TO HAVE A FIXED
STOP ON THE THROTTLE PEDAL.
This will reduce wear on the throttle
cable and carburetor. Remove the
air filter and visually confirm that the
throttle slide reaches the wide open
position just as the pedal hits the stop.
The slide should also fully close with
some slack in the cable with the pedal
released. Idle speed cannot be set if the
pedal is holding the slide partially open.
Seal the pedal end of the cable housing
from dirt. Dirt will migrate up the
throttle housing especially on a dirt
track. Grit or debris on the slide could
cause sticking. A rubber cap can be
used on the end of the cable housing
with a small hole through it for the
cable to slide through. A dab of silicone
could also be used at the end of the
cable housing. The cable housing
should also be sealed by the rubber
boot on the top of carburetor cap.
For the cable hook-up, remove the
cover, spring and slide assembly. USE
CAUTION while unscrewing the cover
as it will have spring tension behind it.
Make a note of the orientation of the
slide valve. The small groove on the
side will align with a small guide pin in
the bore. Thread the cable first thru the
boot and cover, then thru the center of
the spring and finally thru the top of the
slide and hook the ball end under the
bottom of the slide. (See photo.)
The hose attached to the bottom of
the carburetor is for fuel overflow from
the bowl. It should be routed down for
proper draining.
Fuel pressure requirements are 1-3 psi.
