Jeep J-10 Series 1982 Technical & Service Manual page 374

1J-62 FUEL SYSTEMS
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 that flows through the
power valve is added to the fuel in the main metering
circuit to enrich the mixture. As engine load require
ments decrease, manifold vacuum increases and over
comes the tension of the power valve spring, closing the
power valve.
Pump Circuit
When the throttle valves are opened quickly, the air
flow through the carburetor responds almost immedi
ately. Because fuel is heavier than air, there is a brief
lag in time before the fuel flow can gain sufficient veloc
ity to maintain the proper air/fuel mixture ratio. Dur
ing this lag, the pump circuit supplies the required fuel
until the proper air/fuel mixture ratio can be main
tained by the other metering circuits (fig. 1J-121).
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.
SPRING
DIAPHRAGM
80007
Fig. 14-121 Pump Circuit
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.
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.
A vent 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 and idle metering circuits. This provides the richer
air/fuel mixture required for cold engine starting.
The choke shaft is connected by linkage to a bimetallic
coil that winds up (contracts) when cold and unwinds
(expands) when warm.
The position of the choke valve is controlled by the
action of a vacuum modulator that exerts force against
the tension of the bimetallic coil.
The altitude compensation circuit has a separate non-
adjustable choke valve that is linked directly with the
primary choke valve.
When the engine is cold, tension of the bimetallic coil
holds the choke valve closed. When the engine is started,
manifold vacuum is channeled through an opening at
the base of the carburetor through a passage on the
bottom side of the modulator diaphragm assembly, to
move the diaphragm downward against the setscrew. At
the same time, the modulator arm contacts a tang on the
choke shaft. The downward movement of the diaphragm
assembly compresses the piston spring and exerts a
pulling force on the modulator arm, causing the choke
valve to open slightly. This opening is referred to as
initial choke valve clearance.
The bimetallic coil is warmed by electric heater ele
ment and, as the engine begins to warm up, heated air
routed from the exhaust crossover through a heat tube
to the choke housing. A thermostatically controlled by
pass valve, which is an integral part of the choke heat
tube, helps prevent premature choke valve opening dur
ing the early part of the engine warmup period. The
valve regulates the temperature of the hot airflow to the
choke housing by allowing outside unheated air to enter
the heat tube. A thermostatic disc is incorporated in the
valve. It is calibrated to close the valve at 75°F (24°C)
and open it at 55°F (13°C).
The electric heater element and heated air entering
the choke housing cause the bimetallic coil to begin
unwinding and decrease the closing tension exerted
against the choke valve. The coil gradually loses its
tension and allows the choke valve to open.
When the engine reaches normal operating temper
ature, the bimetallic coil continues unwinding and
exerts pressure against the choke linkage, forcing the
choke valve fully open. A continual flow of heated air
passes through the choke housing and is exhausted into
the intake manifold. The bimetallic coil remains heated
and the choke valve remains fully open until the engine
is stopped and allowed to cool.