Holley MCarbHoll2110 1957 Carburetor manual
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As the throttle opening is suddenly increased
for acceleration, the air flow through the carbu
retor responds almost immediately.
Since fuel,
however, is heavier than air, there is a brief
interval before fuel flow responds to the increase
in the throttle opening. During that short period,
since the desired balance of fuel and air cannot
be maintained by the other fuel metering systems,
the accelerating pump becomes functional, sup
plying fuel until the other metering systems can
provide the proper mixture.
The accelerating pump, connected to the
throttle lever by the pump link, is actuated by
throttle movement. When the throttle is moved
toward the closed position, the pump piston is
raised by the linkage, charging the accelerating
pump well with fuel. This upward movement of
the piston draws fuel from the float chamber
through the pump inlet and into the pump well.
The pump inlet check valve opens to permit fuel
to enter the pump well and closes to prevent a
reverse flow of fuel when the pump is in operation.
As the throttle is opened, the pump rod is drawn
downward by the pump link. When the pump rod
moves downward, the horizontal arm in its upper
end slides down in the slot in the piston stem,
compressing the pump spring.
This over-riding
feature of the pump discharge valve provides a
prolonged, even discharge of fuel, for smooth ac
celeration. The pressure of the compressed
spring moves the pump piston downward. This
forces the fuel, in the bottom of the pump well,
out through the pump discharge passage, raising
the pump discharge needle valve. The fuel flows
around the needle and is metered through the
calibrated pump discharge nozzle into the air
stream entering the venturi. The pump discharge
valve prevents the syphoning of fuel f rom the pump
well at high speeds, also prevents air from enter
ing the pump chamber on the loading stroke.
A larger or smaller accelerating pump dis
charge, to compensate for climatic conditions,
can be obtained by varying the position of the pump
link pin in the holes provided in the throttle lever.
As the engine is running, the intake stroke
of each piston draws the air through the carbu
retor venturi and booster venturi. The air, passing
through the reduced area of the venturi, creates
a low pressure commonly called a vacuum. The
strength of this low pressure is determined pri
marily by the velocity of the air flowing through
the venturi. This, in turn, is regulated by the
speed and power output of the engine.
The dif
ference, between the pressure in the venturi and
the normal air pressure in the float chamber,
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