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10-52
GROUP
10- FUEL SYSTEM
Fig. 8 - High-Speed Air Bleeds
Air Bleed Check Valve
With the
engine operating
at high
speed, a vacuum
exists
at the accele-
rating pump discharge nozzles. An
air bleed
check
valve prevents this
vacuum from siphoning fuel through
the accelerating pump system when
fuel is not being
discharged.
The
valve is placed at the upper
end
of
the discharge nozzle passage.
POWER FUEL SUPPLY SYSTEM
The main metering
system
provides
a lean mixture for cruising condi-
tions, when power requirements are
not high. When more power is re-
quired for high speed operation or
for accelerating it is necessary to
burn more fuel. The small amount of
fuel
in
a lean mixture does not pro-
vide
enough
heat upon combustion
for full
engine
power. Therefore, a
way to
"step
up" or richen the mix-
ture is provided
..
.
this
"step
up"
system is termed the power fuel
supply system (Fig.
11).
Vacuum Piston and Power Valve
The power fuel
supply system
uses
a vacuum-controlled piston
in
the air
horn body and a power valve to admit
more fuel when power
is
required.
The vacuum piston rod is
spring-
PARTIAL
VACUUM
INTAKE CHECK VALVE (UNSEATED)
.
FUEL
CLOSED
Fig. 10- Accelerating Pump- Discharge
loaded, and tends to
push
the
rod
down. The stem
of
the
power valve is
also spring-loaded, tending
to
hold
the power valve up or closed.
Manifold vacuum is
sensed
on top
of
the piston through passages in the
carburetor
bodies. At idle or cruising
conditions, the vacuum is high enough
to
overcome
the p1ston rod sprmg
force. The piston and rod are held
up and away from the power valve
stem.
The power valve
spring
then
holds the
valve
closed.
Another Passage to the Main Well
When the
engine
is under load. the
vacuum
drops.
The vacuum piston
rod is pushed down by its spring and
the rod pushes on the power valve
stem.
The comparatively light power
OAIR
•
FUEL
VACUUM PISTON
MANIFOLD
VACUUM
CHANNEL
POWER
.
FUEL
.
VACUUM
valve
spring
is overcome and the
valve
opens
(Fig.
12). Opening
the
valve gives another passage from the
bowl
to the main wells
..
.
through
the
valve
and power
jets.
The effect
is the same as if a temporary increase
in the
size of
the main jets was made.
More fuel is discharged through the
main metering system to make the
mixture richer.
When the
engine
load decreases,
the process is
reversed.
The piston is
pulled up by higher manifold vacuum
and allows the power valve to
close.
SECONDARY MAIN FUEL
METERING SYSTEM
.
All the
"action"
described in this
section so far is within the primary
POWER JET
Fig. 9 -Accelerating Pump System-Intake
Fig. 11 -Power Fuel Supply System
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