Buick LeSabre 1965 Service Manual page 110

3 - 3 4 2-BARREL ROCHESTER ENGINE FUEL A N D EXHAUST SYSTEMS
MAIN NOZZLE
MAIN WELL,
AIR BLEED
MAIN WELL TUBE-
MAIN WELL INSERT
MAIN METERING
JET
MIXTURE PASSAGE
•BOOST VENTURI
MAIN VENTURI
THROTTLE VALVE
Figure 3-29—High Speed System
PISTON VACUUM
CHAMBER
POWER PISTON
POWER PISTON
SPRING
POWER VALVE
POWER RESTRIC
Figure 3-30—Power System
f. Operation of Accelerating
System
When the throttle valve is opened
rapidly, the air flow and manifold
vacuum change almost instantane-
ously, while the heavier fuel tends
to lag behind causing a momen-
tary leanness. The accelerator
pump provides the fuel necessary
for smooth operation on rapid
acceleration.
Fuel for acceleration is supplied
by a double-spring loaded pump
plunger. The top and bottom
springs combine to move the
plunger so that a smooth, sus-
tained charge of fuel is delivered
for acceleration.
Fuel is drawn into the pump well
through the inlet ball check on the
intake stroke of the pump plunger
(upward stroke). See Figure 3-31.
Downward motion of the pump
plunger seats the inlet ball check
and forces the fuel through the
discharge passage where it un-
seats the pump discharge ball and
then passes on through to the
pump jets where it sprays into
the venturi.
The ball check located in the
pump plunger head serves as a
vapor vent for the pump well.
Without this vent, vapor pressure
in the pump well might force fuel
from the pump system into the
engine manifold causing hard
starting when the engine is hot.
There is an inner hole in the
pump lever to provide a richer
pump adjustment for extreme cold
temperature conditions. This in-
ner hole should be used only when
low temperature hesitation in-
dicates a too lean pump setting.
Figure 3-31— Accelerating
Pump System
The pump discharge ball check in
the accelerator pump passage
prevents any pullover or dis-
charge of fuel from the pump
n o z z l e s when the accelerator
pump is inoperative.
g. Operation of
Choke System
The choke system is composed of
a thermostatic coil, vacuum choke
piston, offset choke valve, fast
idle cam and choke linkage. Its
operation is controlled by a com-
bination of intake manifold vac-
uum, the o f f s e t choke valve,
atmospheric temperature and ex-
haust manifold heat. See Figure
3-32.