Service Manual

3-16 FUEL SYSTEM
Carburetor
FR and FS Models
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 dur-
ing 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 carbu-
retor 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 throt-
tle 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, allow-
ing 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 me-
ter 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 mix-
ture screw) out (counterclockwise) enriches the mixture;
turning it in (clockwise) leans the mixture.
Main Fuel Flow →
Pilot Fuel Flow ⇒