Holley MCarbHoll2110 1957 Carburetor manual
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causes fuel to flow through the main metering
system. The fuel moves up the main well, passed
main well air bleed where air is added, the amount
of air increasing proportionally as the speed in
creases. This mixture of fuel and air, being
lighter than raw fuel, has a quicker response to
changes in the venturi pressure, and also vaporizes
more readily than raw fuel when discharged into
the venturi air stream. The mixture of fuel and
air flows through the horizontal channel and out
the discharge nozzle into the booster venturi and
then into the air stream in the carburetor venturi.
The throttle plates control the amount of fuel air
mixture to the intake manifold, regulating the
speed and power output of the engine in accordance
with accelerator pedal movement.
During high power operation, the carburetor
must provide a mixture richer than is needed
when the engine is running at cruising speed under
no great power requirements. The added fuel for
power operation is supplied by the power enrich
ment system, sometimes referred to as the
economizer system.
This system is controlled by manifold vacu
um which gives an accurate indication of the
power demands placed upon the engine. Manifold
vacuum is strongest at idle and decreases as the
load on the engine increases. As the load on the
engine is increased, the throttle plate must be
opened wider to maintain a given speed. Manifold
vacuum is thus reduced because the opened throt
tle plate offers less restriction to air entering the
intake manifold.
A vacuum passage in the throttle body trans
mits manifold vacuum to the power valve diaphragm
in the vacuum chamber. The manifold vacuum,
acting on the diaphragm at idle or normal load con
ditions, is strong enough to hold the diaphragm
down, overcoming the tension of the power valve
spring on the stem. This holds the valve closed.
When high power demands place a greater loadon
the engine and manifold vacuum drops below a pre
determined point, the power valve spring over
comes the reduced vacuum and expands, opening
power valve. Fuel from the float chamber flows
through the valve and out the small holes in the
of the valve into the passages leading to both main
wells. A restriction in the passage leading to each
main well controls the amount of fuel the power
valve meters, thus insuring an even distribution of
fuel. In each main well, the fuel joins the fuel flow in
the main metering system, enriching the mixture.
As engine power demands are reduced, mani
fold vacuum increases. The increased vacuum
acts on the diaphragm, and the diaphragm over
coming the tension of the power valve spring,
moves downward. This closes the power valve
and shuts off the added supply of fuel which is no
longer required.
The spark control valve which is located in
the throttle body provides an efficient degree of
spark during periods of acceleration by momen
tarily providing an intermediate spark advance
curve between a fully retarded spark and the
normal spark advance which would be excessively
retarded when the engine is suddenly accelerated.
The spark control valve accomplishes this by
controlling the manifold vacuum admitted to the
distributor vacuum line which would bleed off the
higher, venturi vacuum at wide open throttle.
All manifold vacuum to the distributor passes
through the spark control valve. Under normal
road conditions, the spark valve is held open by a
combination of pressure; atmospheric outside the
diaphragm, and manifold vacuum from within. The
combined forces act against the tension of a preset
factory calibrated spring.
When the manifold
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