Service Manual

3-8 FUEL SYSTEM
Carburetor
Fuel and Air Flow
The main system of the carburetor consists of the main jet [A],
main nozzle [B], and the main air passage [C] (main air jet [D]). The
main system meters fuel to the engine during moderate to heavy load
conditions. Fuel flows through the main jet and into the main nozzle,
where it is joined by air from the main air passage (main air jet). The
resulting mixture flows out the end of the main nozzle into the carburetor
bore, where it is atomized by the high speed air flow, and carried into
the engine.
The pilot system includes the pilot jet [E], pilot screw [F] (Idle mixture
screw), pilot air jet [G], pilot outlet [H], and the bypass holes [I]. The pilot
system meters the fuel/air mixture while the engine is idling and running
under a light load. Under these conditions there is very little air flow
through the carburetor bore; so little that it is not enough to draw fuel
through the main system of the carburetor and atomize it. Instead, the
fuel is drawn through the pilot system, since the nearly closed throttle
valve [J] causes high speed air flow past the pilot outlet and bypass
holes (even at low engine speed).
Fuel flow in the pilot system is metered by the pilot jet. Air for better
atomization is admitted via the pilot air jet in the mouth of the carburetor.
The fuel/air mixture passes into the bore of the carburetor side stream
of the throttle valve through the bypass holes and pilot outlet. While
the throttle valve is almost closed, it covers the small bypass holes
opening into the bore from the pilot system. As the throttle valve begins
to open, it uncovers the bypass holes, allowing more fuel/air mixture to
flow. The extra flow is needed because the engine starts to run faster
as the throttle is opened. The pilot screw controls the amount of fuel/air
mixture allowed through the pilot outlet, but does not meter the bypass
holes. A moderate amount of air comes in around the throttle valve
at idle, so adjusting the pilot screw changes the fuel/air ratio. Turning
the pilot screw (Idle mixture screw) out (Counterclockwise) enriches the
mixture; turning it in (clockwise) leans the mixture.
Main Fuel Flow
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Pilot Fuel Flow ⇒