Service Manual

FUEL SYSTEM 3-15
Carburetor
The pilot system includes the pilot jet [F], pilot screw [G] (Idle mixture screw), 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 an idle, so adjusting the pilot
screw changes the fuel/air ratio. Turning the pilot screw (Idle mixture screw) out (Counterclockwise)
enrichens the mixture; turning it in (clockwise) leans the mixture.