5. AIR
BLEEDS - Figure 4
The
fuel supplied to the cylinders must be vaporized
to burn completely during the power stroke of
the piston. Fuel which enters the cylinders
in liquid form burns too slowly and is wasted.
Vaporization of the fuel discharge is aided
by air bleed passages which introduce air into
the stream of fuel before it is discharged.
This emulsion of fuel and air responds more
readily to any change in vacuum and vaporizes
more efficiently when it is discharged.
6. THE
THROTTLE - Figure 4
The
throttle plate in the carburetor bore governs
the power output of the engine by regulating
the amount of fuel air mixture admitted to the
intake manifold. It is controlled by the driver
of the vehicle through the accelerator pedal.
7. THE
FLOAT SYSTEM - Figure 4
The
float system provides a constant supply of fuel
in the float chamber for use by the fuel metering
systems in the carburetor. Fuel under pressure
from the fuel pump enters the
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float chamber through the
fuel inlet needle valve. The float, which rises
or lowers with the fuel level in the float chamber,
controls the fuel inlet needle valve to admit
only enough fuel to replace that being used.
When
the engine is started, the fuel level in the float
chamber begins to drop as fuel is used. The float
is lowered, opening the fuel inlet needle valve
and allowing more fuel to enter the float chamber.
When the fuel in the float chamber rises to a
specified level, the needle valve will restrict
the flow of fuel into the float chamber so that
only enough fuel is admitted to replace that being
used, thus maintaining a constant fuel level.
Actually, the fuel level will drop slightly as
engine speed increases, since the needle valve
must be opened more to meet the increased fuel
demands of the engine.
8. LOW
POWER OPERATION
At idle
and low speeds, the air flow through the carburetor
is greatly reduced and the vacuum in the venturi
is too weak to draw fuel from the main metering
system. The nearly closed throttle plate restricts
the flow of air into the engine, resulting in
a strong manifold vacuum.
9. THE
IDLE SYSTEM-Figure 5
During
low power operation, the pressure difference between
the manifold and the float chamber forces fuel
through the idle system. The fuel flows through
the main metering jet into the bottom of the main
well, where it is drawn upward through the idle
tube. The narrow tip at the bottom of the idle
tube is a calibrated restriction which primarily
meters the flow of fuel in the idle system. (An
exception is the Holley model 859 updraft carburetor
which contains no idle tube. In this carburetor,
the idle system fuel flow is metered by a pressed-in
restriction in the idle passage.)
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