Pump Circuit (2100-2 Venturi); Choke Circuit (2100-2 Venturi) - Jeep 1979 CJ-5 Technical & Service Manual

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Manifold vacuum is applied to the power valve dia-
phragm from an opening in the base of the main body,
through a passage in the main body and power valve
chamber to the power valve diaphragm. During idle and
normal driving conditions, manifold vacuum is high
enough to overcome the power valve spring tension and
holds the valve closed. When higher engine output is
required, the increased load on the engine results in
decreased manifold vacuum. The power valve spring
opens the first stage of the power valve when manifold
vacuum drops below a predetermined value and a pmall
amount of fuel flows through the valve.
When manifold vacuum drops to a lower value, the
power valve spring opens the second stage of the power
valve and allows a greater amount of fuel to flow
through the valve.
The fuel which flows through the power valve is added
to the fuel in the main metering circuit to enrich the
mixture. As engine load requirements decrease, mani-
fold vacuum increases and overcomes the tension of the
power valve spring, closing the power valve.
8 0 0 0 8
Fig. 1J-43 Power Enrichment Circuit
Pump Circuit
When the throttle valves are opened quickly, the air-
flow through the carburetor responds almost immedi-
ately. Since fuel is heavier than air, there is a brief lag in
time before the fuel flow can gain sufficient speed to
maintain the proper fuel-air ratio. During this lag, the
pump circuit supplies the required fuel until the proper
fuel-air ratio can be maintained by the other metering
circuits (fig. 1J-44).
The pump is charged when the throttle valves are
closed. - The diaphragm return spring exerts force
against the diaphragm and pushes it against the cover.
FUEL SYSTEMS 1J-23
Fuel is drawn through the inlet, past the Elastomer
valve, and into the pump chamber. A discharge check
ball and weight prevents air from being drawn into the
pump chamber.
Fig. 1J-44 Pump Circuit
When the throttle valves are opened, the diaphragm
rod is pushed inward forcing fuel from the pump cham-
ber into the discharge passages. The Elastomer valve
seals the inlet hole during pump discharge preventing
fuel from returning to the fuel bowl. Fuel under pres-
sure unseats the discharge check ball and weight and is
forced through the pump discharge screw. The fuel is
then sprayed into the main venturi through discharge
ports.
An air bleed is provided in the pump chamber to
prevent vapor accumulation and pressure buildup.
Choke Circuit
The choke valve, located in the air horn assembly,
provides a high vacuum above as well as below the
throttle valves when closed. During cranking, vacuum
above the throttle valves causes fuel to flow from the
main metering and idle circuits. This provides the richer
fuel-air mixture required for cold engine starting.
The choke shaft is connected by linkage to a thermo-
static coil which winds up when cold and unwinds when
warm.
The position of the choke valve is controlled by the
action of a vacuum modulator exerting force against the
tension of the thermostatic coil (fig. 1J-45).
When the engine is cold, tension of the thermostatic
coil holds the choke valve closed. When the engine is
started, manifold vacuum is channeled through an open-
ing at the base of the carburetor through a passage of
the bottom side of the modulator diaphragm assembly,
moving the diaphragm downward against the set screw.