Main Metering (High Speed) Circuit; Power Enrichment; Power Enrichment Valve - Jeep J-10 Series 1982 Technical & Service Manual

venturi. The air inlets serve as anti-siphon vents during
off-idle, high speed operation and when the engine is
stopped.
The air/fuel mixture moves down the idle passages
past the idle transfer slots, which serve as additional air
inlets during curb idle operation. The air/fuel mixture
then moves past the idle mixture adjustment screw tips,
which control the amount of discharge. From the adjust
ment screw ports, the air/fuel mixture moves through
short horizontal passages and is discharged below the
throttle valves.
At speeds slightly above idle, the idle transfer slots
begin discharging the air/fuel mixture as the throttle
valves expose them to manifold vacuum. As the throttle
valves continue opening and engine speed increases, the
airflow through the carburetor increases proportion
ately. This increased airflow creates a vacuum in the
venturi and the main metering circuit begins dis
charging- the air/fuel mixture. The discharge from the
idle circuit tapers off as the main metering circuit be
gins discharging.
Main Metering (High Speed) Circuit
As engine speed increases, the air velocity through the
booster venturi creates a low pressure area. Fuel flow
through the main metering circuit is caused by atmos
pheric pressure in the fuel bowl and a lower pressure at
the main discharge ports. Fuel flows from the fuel bowl,
through the main jets and into the main wells. The fuel
then moves up the main well tubes where it is mixed
with air. The air, supplied through the main air inlets,
mixes with the fuel through small holes in the sides of
the main well tubes. The main air inlets meter an in
creasing amount of air, whenever venturi vacuum in
creases, to maintain the proper air/fuel mixture ratio.
The mixture of fuel and air, being lighter than raw fuel,
responds quickly to changes in venturi vacuum. It also
atomizes more readily than raw fuel.
The air/fuel mixture moves from the main well tubes
to the discharge ports and is discharged into the booster
venturi (fig. 1J-119).
Anti-siphon air vents, located near the top of the main
well tubes, prevent siphoning of fuel from the main well
when decelerating.
Power Enrichment
During heavy load conditions or high speed operation,
the air/fuel mixture ratio must be increased to provide
higher engine power output. The power enrichment
valve supplies extra fuel during this period. It is con
trolled by intake manifold vacuum (fig. 1J-120).
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
FUEL SYSTEMS 14-61
U ANTI-SIPHON
^ AIR BLEED
S
Fig. 14-119 Main Metering Circuit
80006
80008
Fig. 14-120 Power Enrichment Valve
chamber to the power valve diaphragm. During engine
idle and cruise speed conditions, manifold vacuum is
high enough to overcome the power valve spring tension
and holds the valve closed. When higher engine power
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 small amount of fuel flows through the valve.