Lawn-Boy M Series Service Manual page 21

OPERATION
Float
Opening the fuel shut-off valve allows fuel to
flow from the fuel tank to the fuel inlet on the
carburetor. The fuel cap is vented to allow air to
enter the fuel tank. I f the vent is plugged the fuel
flow may stop, causing the engine to stop.
Fuel flows through the inlet needle seat
and into the carburetor bowl. As the fuel level
rises in the carburetor bowl the inlet needle
valve is lifted by the float, rising until it seats in
the inlet needle seat, stopping the fuel flow. As
fuel is used, the float will move downward
allowing more fuel to enter the carburetor bowl
through the inlet needle seat.
The float and inlet needle maintain a constant
fuel level in the carburetor bowl. A bowl vent
allows atmospheric air to enter the bowl.
If
this
vent is blocked, the fuel flow will change, caus-
ing poor engine performance.
Idle Speed
When the engine is operating with the throttle
lever in the "slow" position, the fuel flow is as
follows:
When the intake port is opened by the
piston skirt, a partial vacuum is created inside
the engine crankcase. Atmospheric pressure in
the carburetor bowl pushes fuel through the
main jet and into the idle passage. At the same
time atmospheric pressure pushes air into the
throat of the carburetor allowing air to enter
through the pilot air jet. Fuel forced up the idle
passage enters the center of the pilot jet, mixing
with air from the pilot air jet.
This air/fuel mixture then is routed to the pri-
mary idle port and out into the carburetor where
it combines with the incoming air through the
carburetor throat and into the intake port
of
the
engine.
As engine speed is increased, the throttle
valve opens and the air/fuel mixture flow in-
creases and flows out the primary and secon-
dary idle ports.
Throttle Pilot Jet
Pilot
Jet
Primary
Idle Port
Secondary
Idle Port
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