Power Enrichment Circuit; Pump Circuit; Choke Circuit - Jeep 1979 CJ-5 Technical & Service Manual

Fig. 1J-18 idle Circuit
normal pressure, fuel flows to the lower pressure area
created by the venturi and magnified by the booster
venturi.
The fuel flow moves through the main jets to the main
well. Air enters through the main well air bleeds. The
resulting mixture of fuel and air is lighter than raw fuel,
responds more quickly to changes in venturi vacuum,
and is more readily vaporized when discharged into the
venturi (fig. 1J-19).
Power Enrichment Circuit
During heavy road load or high speed operation, the
fuel-air ratio must be enriched to provide increased en-
gine power.
Power enrichment is accomplished by means of two
calibrated metering rods yoked to a single manifold vac-
uum actuated piston (fig. 1J-19). The metering rod pis-
ton rides on a calibrated spring which attempts to keep
the piston at the top of its cylinder. This allows only the
smallest diameter of the tapered metering rods to ex-
tend into the main metering jets and permits maximum
fuel flow through the jets to the main well cavities.
At idle, part throttle or cruise conditions when mani-
fold vacuum is high, the piston is drawn down into the
vacuum cylinder against calibrated spring tension and
the larger diameters of the metering rods extend into
the main metering jets, restricting the fuel flow to the
main well cavities. An additional control is provided by
the rod lifter on the accelerator pump rod. This provides
a direct relationship between metering rod position and
throttle valve opening.
Pump Circuit
When the throttle is opened suddenly, airflow re-
sponse through the carburetor is almost immediate.
FUEL SYSTEIS 1J-11
Fig. 1J-19 Main Metering Circuit
There is a brief time lag before fuel inertia can be
overcome. This lag causes the desired fuel-air ratio to be
leaned out.
A piston-type accelerating pump system mechanically
supplies the fuel necessary to overcome this deficiency
(fig. 1J-20).
Fuel is drawn into the pump cylinder from the fuel
bowl through a port and check ball in the bottom of the
pump well below the pump piston. When the engine is
turned off, fuel vapors in the pump cylinder vent
through the area between the pump rod and pump
piston.
As the throttle lever is moved, the pump link, oper-
ating through a system of levers and assisted by the
pump drive spring, pushes the pump piston down. Fuel
is forced through a passage, past the pump discharge
check ball, and out the pump discharge jets in the ven-
turi cluster.
Choke Circuit
The choke valve, located in the air horn assembly,
provides a high vacuum both above and below the
throttle valves when closed. During cranking, vacuum
above the throttle valve causes fuel to flow from the
main metering and idle circuits and provides the richer
fuel-air ratio needed for cold engine starting (fig. 1J-21).
The choke shaft is connected by linkage to a thermo-
static coil within the choke cover, which winds up when
cold and unwinds when heated. When the engine is cold,
the tension of the thermostatic coil holds the choke valve
closed. When the engine starts, manifold vacuum is ap-
plied to the diaphragm assembly to open the choke valve
slightly. This is called the initial choke valve clearance.