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smooth, even transition from idle to cruising speeds.
At the idle and low speeds, the velocity of the air
flowing through the carburetor is reduced and the
vacuum created in the venturi will not be strong enough
to operate the main metering system. Because of the
restriction of the air flow through the carburetor due to
the nearly closed throttle plate, intake manifold vacuum
will be high. This high manifold vacuum provides a
pressure differential which is used to operate the idle
At the idle, fuel flows through the main jet into the
bottom of the main well. The high manifold vacuum
acting on this fuel through the idle system passages
draws the fuel from the main well through a short
horizontal passage into the idle well. A calibrated
restriction in the lower portion of the idle well meters the
flow of fuel entering the idle system. The fuel passes out
the top of the idle well and into the idle system passages
in the main body. A metered flow of air from the idle air
bleed is admitted to the fuel as it enters the idle passage
in the main body. The idle air bleed also serves to vent
the idle system to prevent any siphoning effect at higher
speeds or when the engine is stopped. This mixture of
fuel and air continues down, flowing through the idle
restriction and, passing the two idle transfer holes in the
throttle body, is discharged through the idle discharge
hole into the strong manifold vacuum existing below the
throttle plate. The two idle transfer holes act as additional
air bleeds at the idle. An idle adjusting needle, which
seats in the idle discharge hole, controls the discharge of
fuel at the idle and provides a means for adjusting the
idle mixture of the engine. Turning the idle adjusting
needle in moves the pointed tip of the needle closer to its
seat, restricting the fuel flow out of the idle discharge
hole. This results in a leaner idle mixture. Conversely,
turning the needle out allows more fuel to flow out the
idle discharge hole to provide a richer idle mixture.
During off-idle operation, which occurs when the throttle
plate is moved open slightly past the two idle transfer
holes, each hole begins discharging fuel as it is exposed to
manifold vacuum. As the throttle plate is opened still
wider and engine speed increases, the velocity of the air
flow through the carburetor is also increased. This creates
a vacuum in the venturi strong enough to bring the main
metering system into operation. The flow from the idle
system tapers off as the main metering system begins
discharging fuel. The two systems are engineered to
provide a
When high power output is required, a richer mixture must be
provided than is required for normal cruising when no great
load is placed on the engine. The carburetor provides the added
fuel for high power operation by means of the power
enrichment system, sometimes called the economizer system.
The power enrichment system is actuated by manifold
vacuum. Manifold vacuum, which is strongest at the idle
when there is no load on the engine, is reduced in proportion
to the increase in engine loading. This is due to the fact that,
as the load on the engine is increased, the throttle plate must
be opened wider to maintain any given speed. Manifold
vacuum will be reduced because the restriction offered to
the air flow entering the intake manifold by the throttle plate
will be lessened as the plate is opened. The strength of the
manifold vacuum is thus an accurate indicator of the power
demands placed on the engine..
Manifold vacuum acting on the economizer diaphragm
actuates the power enrichment system. This vacuum from the
lower portion of the throttle bore below the throttle plate is
transmitted through the vacuum passage to the vacuum chamber
on top of the economizer diaphragm. At idle and normal
cruising speeds, the vacuum acting on the economizer
diaphragm is strong enough to hold the diaphragm up against
the tension of the diaphragm. spring. This raises the economizer
diaphragm stem clear of the power valve and the power valve
will be held in the closed position by the tension of its spring.
The power enrichment system will thus be inoperative in
conditions of high manifold vacuum.
When high power demands place a greater load on the engine,
manifold vacuum is reduced. When the vacuum is reduced
below a predetermined point, the diaphragm can no longer
overcome the tension of the diaphragm spring and the stem
will be forced down. This depresses the pin in the center of
the power valve, opening the valve. Fuel from the float
chamber will flow into the valve and, passing through a
horizontal passage, enter the main well. There it is added to
the fuel flow of the main metering system, enriching the
mixture for full power. The drilled plug in the passage
between the power valve and the main well is a calibrated
restriction which meters the flow of fuel through the power
enrichment system.
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