Buick LeSabre 1965 Service Manual page 118

3 - 4 2 4-BARREL ROCHESTER
ENGINE FUEL AND EXHAUST
VACUUM VENT
AUXILIARY
VALVE
POWER PISTON
HIGH
SPEED
BLEED
MAIN DISCHARGE NOZZLES
HIGH
SPEED
BLEED
MIXTURE
DOWN
TUBES
POWER VALVE
POWER RESTRICTIONS PRIMARY
T
n
B f t T T I
P VA. V/F
THROTTLE VALVE THROTTLE VALVE
DOWN TUBE OPERATION
Figure 3-44—Main Metering and Power Systems
slashed on the bottom to provide
a smoother transition between the
opening of the secondary valves
and the opening of the auxiliary
valves. When the air flow is high
enough to open the a u x i l i a r y
valves, the down tubes no longer
feed the fuel because the low
pressure point is now in the small
v e n t u r i . With this feature the
correct fuel-air mixture is sup-
plied at any point during second-
ary throttle valve operation.
e. Operation of
Power System
For maximum power under load
or for all speeds above approxi-
mately 85 MPH, a richer mixture
is required than that necessary
for normal throttle opening. This
additional fuel is provided by one
power system connected to the
main metering systems in the
primary section of the carbu-
retor. See Figure 3-44.
The power piston cylinder in the
air horn of the carburetor is
connected by a channel to the
face of the mounting flange so
it is subject to intake manifold
vacuum. At part throttle position
the vacuum is sufficient to hold
the power piston in its "up" po-
sition against the tension of the
piston spring. When the throttle
valves are opened to a point where
manifold vacuum drops to approx-
imately 9 to 5 inches of mercury
and additional fuel is required for
satisfactory operation, the piston
spring moves the power piston
down to open the power valve.
This allows additional fuel to
enter the main discharge nozzles
in the primary section through
calibrated restrictions l o c a t e d
below the main metering jets.
See Figure 3-44.
f. Operation of
Accelerating System
For smooth and rapid accelera-
tion it is necessary to supply an
extra quantity of fuel momen-
tarily when the throttle is opened
suddenly. This is accomplished
by one accelerating pump piston
which is directly connected to
the primary throttle shaft lever
by means of a rod and pump
lever.
When the throttle is closed, the
pump piston moves up and draws
a supply of fuel from the float
chamber through the inlet strain-
er, past the inlet ball check valve
and into the pump cylinder. When
the throttle is opened, the piston