3-13
Fuel System
3-13
MAIN FUEL SYSTEM
The velocity of air flow through the carburetor
increases as the throttle plate opens. When this causes a
great enough pressure drop in the venturi the main metering
system starts flowing. Fuel from the idle system tapers off as
the main system begins discharging fuel. At this time, there is
a definite blend of the idle and main systems.
Fuel passes through the main Jet into the bottom of
the main well and flows up the main well (Figure 27). Filtered
air from the carburetor air inlet passes through the high speed
air bleed into the air bleed well and enters the fuel in the main
well through three short horizontal air passages. This mixture
of fuel and air, being lighter than raw fuel, responds faster to
any change in venturi pressure and also vaporizes more
readily when it is discharged. The fuel continues up the main
well and flows into the main discharge nozzle where it is
sprayed onto the open choke plate and the walls of the
booster venturi. Here, the mixture Is vaporized and mixed with
the air stream passing through the booster venturi. It then
FIG. 28. Accelerating System
passes the throttle plate into the intake manifold.
POWER FUEL SYSTEM
The power fuel system (Figure 29) operates when
additional fuel is required for a richer mixture during high
speed, heavy loads, and for low speeds at full throttle.
Manifold vacuum is transmitted from below the
throttle plate through the vacuum passage to the vacuum
chamber on top of the diaphragm. At idle and normal speeds
the manifold vacuum is great enough to hold the diaphragm up
against the tension of the diaphragm spring. This raises the
diaphragm stem clear of the power valve. The power, . valve
is held closed by the tension of its spring
When high power places a greater load on the
engine, manifold vacuum is reduced. When the vacuum drops
below 7-6 inches of mercury, the diaphragm can no longer
overcome the tension of the diaphragm spring and the
diaphragm stem is forced down on the power valve This
FIG. 27. Main Fuel System
depresses the pin in the center of the power valve. opening
ACCELERATING SYSTEM
the valve Fuel from the float chamber flows into the valve and
During periods of sudden acceleration, the air flow
passes through a restriction into a horizontal passage which
through the carburetor responds very quickly to a sudden
leads to the main well A here It is added to the fuel from the
throttle opening. However, there is a brief interval before the
main fuel system
heavier fuel-air mixture in the narrow passages can gain
speed and maintain the desired balance of fuel and air. The
ADJUSTMENTS
accelerating system (Figure 28) operates during this interval to
IDLE FUEL MIXTURE ADJUSTMENT
supply fuel until the other systems can provide the proper
The idle fuel mixture is controlled by the idle mixture
mixture.
adjustment needle (Figure 30). Turn the screw "in" to lean the
When the throttle is suddenly opened, the diaphragm,
mixture, and "out" to enrich the mixture. Make the initial
which is connected by linkage to the throttle, forces fuel from
mixture adjustment by turning the needle "in" until it lightly
the pump chamber into the pump discharge passage The fuel
touches the seat Then back off the screw one turn.
under pressure forces the pump discharge ball check valve
Do not turn the needle against the seat tight enough to
and weight up. The fuel then passes into the pump discharge
groove the point. If the needle is damaged, it must be
nozzle where it is sprayed into the air stream of the venturri.
replaced before proper mixture adjustment can be
The discharge nozzle is vented to prevent siphoning at high
obtained.
engine speeds.
Run the engine for 20 minutes at fast Idle speed to
When the throttle is closed, the pump return spring
bring it to normal operating temperature
forces the pump diaphragm toward the back of the pump
Turn the mixture adjusting needle in until the engine
chamber. drawing fuel into the chamber through the pump
begins to run rough from the lean mixture. Slowly turn the
inlet. A ball check valve in the pump inlet opens to admit fuel
needle out until the engine begins to "roll" from the rich
from the float chamber and closes when the pump is operated
mixture.
to prevent a reverse flow of fuel. The outlet ball check valve
prevents air from entering when the diaphragm draws fuel into
the pump chamber.
