FUEL AND EXHAUST SYSTEMS
float arm is a float needle, which rides inside
the float valve. As the float rises, the float
needle rises inside the float valve and blocks it,
so that when the fuel has reached the required
level in the float bowl, no more can enter.
Pilot and Main Fuel Systems
The carburetor's purpose is to supply and
atomize fuel and mix it in correct proportions
with air that is drawn in through the air intake.
At primary throttle openings (from idle to 1/8
throttle), a small amount of fuel is siphoned
through the pilot jet by suction from the
incoming air. As the throttle is opened further,
the air stream begins to siphon fuel through the
main jet and needle jet. The tapered needle
increases the effective flow capacity of the
needle jet as the needle rises with the slide,
because it occupies less of the needle jet's area.
In addition, the amount of cutaway in the
leading edge of the throttle slide aids in
controlling the fuel/air mixture during partial
At full throttle, the carburetor venturi is fully
open and the needle is lifted far enough to
permit the main jet to flow at full capacity.
The choke system consists of a sliding choke
valve and a vacuum relief plate on the choke
valve. When the choke valve closes the
carburetor opening it causes a very high
vacuum in the carburetor bore. Fuel flows from
all jets and mixes with air coming through the
relief plate opening to provide a rich mixture
for cold starting.
If the bike stalls or bogs down under hard
acceleration, check the fuel level as described
in this chapter.
Ifthe mixture is too lean at any or all throttle
settings. the engine may overheat. It may
generate brown exhaust smoke. It may stutter
at high rpm. The performance (acceleration
and top speed)
will fall off.
You may be able to
confirm this by checking the spark plugs. Ifthe
mixture is too lean across the rpm scale, the
spark plugs will be white and their electrodes
may be rounded. While riding the motorcycle.
use the choke to see if the performance
improves with what would normally be an
If the mixture is too rich at any or all throttle
settings. the engine may be sluggish and
blubbery. I t may generate black exhaust
smoke. It may perform best while still cold. If
the mixture is too rich across the rpm scale. the
spark plugs may be black and sooty. Take out
the air cleaner element and ride the motorcycle
to see if the performance improves with what
would normally be too lean a mixture. If it
does. the mixture is too rich. Reinstall the air
filter element immediately to prevent engine
damage from dust and dirt.
Diagnosing the Problem
The fact that the mixture being burned is too
rich or too lean does not necessarily indicate
that the carburetion is at fault. The motorcycle
may as easily have an ignition or compression
If the mixture is wrong all up and down the
rpm range. check the obvious fuel system
components. For example. ifthe mixture is too
rich, check for a clogged air cleaner element or
too high a fuel level in the float bowl. If the
mixture is too lean, check the fuel tap strainer
and the fuel lines for blockage, check for too
low a fuel level and check for an air leak at the
rubber carburetor holders.
Before taking apart the carburetors. you
should first check out the spark plugs, ignition
timing and cylinder compression.
Miscellaneous Carburetor Problems
Water in the carburetor float bowl and a
sticking slide needle can result from careless
washing of the motorcycle. To remedy the
problem, remove and clean the carburetor
bowl, main jet and any olher affected parts. Be
sure to cover the air intake when washing the
Be sure that the carburetor mounting clamps
If gasoline leaks past the float bowl gasket.
high-speed fuel starvation may occur. Varnish
deposits on the outside of the float bowl are
evidence of this condition.
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