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The purpose of the carburetor is to supply and meter the mixture of
fuel vapor and air in relation to the load and speed of the engine.
Because of engine temperature, speed, and load, perfect carburetion
is very hard to obtain.
The carburetor supplies a small amount of a very rich fuel mixture
when the engine is cold and running at idle. With the throttle plate
closed and air from the air cleaner limited by the closed choke
plate, engine suction is amplified at the idle-circuit nozzle. This
vacuum draws a thick spray of gasoline through the nozzle from the
full float bowl, whose fuel line is closed by the float-supported
needle valve. More fuel is provided when the gas pedal is depressed
for acceleration. The pedal linkage opens the throttle plate and the
choke plate to send air rushing through the barrel. The linkage also
depresses the accelerator pump, providing added gasoline through the
accelerator-circuit nozzle. As air passes through the narrow center
of the barrel, called the "venturi", it produces suction that draws
spray from the cruising-circuit nozzle. The float-bowl level drops
and causes the float to tip and the needle valve to open the fuel
line.
To cause a liquid to flow, there must be a high pressure area (which
in this case is atmospheric pressure) and a low pressure area. Low
pressure is less than atmospheric pressure. The average person refers
to a low pressure area as a vacuum. Since the atmospheric pressure is
already present, a low pressure area can be created by air or liquid
flowing through a venturi. The downward motion of the piston also
creates a low pressure area, so air and gasoline are drawn through
the carburetor and into the engine by suction created as the piston
moves down, creating a partial vacuum in the cylinder. Differences
between low pressure within the cylinder and atmospheric pressure
outside of the carburetor causes air and fuel to flow into the
cylinder from the carburetor.
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