Buick LeSabre 1965 Service Manual page 119

ENGINE FUEL AND EXHAUST
4-BARREL ROCHESTER 3 - 4 3
ACCELERATOR
PUMP
VAPOR
BALL
CHECK
PUMP
DISCHARGE
NOZZLES
OUTLET
CHECK BALL
INLET CHECK BALL
Figure 3-45—Accelerating System
on its downward stroke exerts
pressure on the fuel which closes
the inlet check ball and opens
the outlet check ball. A metered
quantity of fuel is then discharged
through the pump discharge noz-
zles into each barrel in the pri-
mary section of the carburetor.
This o c c u r s only momentarily
during the accelerating period.
The pump duration spring which
is compressed by tne downward
movement of the pump linkage
against the resistance of the fuel
provides a follow-up action so
that the discharge carries out
over a brief period of time. A
ball check in the accelerator pump
plunger acts as a vapor vent to
prevent vapor p r e s s u r e from
forcing fuel from the pump dis-
charge holes during extreme heat
periods. Downward movement of
the plunger, however, seats the
ball and allows normal operation
of the accelerating system. See
Figure 3-45.
When the desired speed is reached
and the throttle is held in a fixed
position, the pressure on the fuel
decreases sufficiently so that the
outlet check ball closes and fuel
ceases to discharge from pump
nozzles. Thus a quantity of fuel
is maintained in the channel ad-
jacent to the outlet check ball
where it is immediately available
for future requirements.
g. Operation of
Choke System
The automatic choke mechanism
is contained in the primary sec-
tion of the carburetor. It consists
of a choke valve mounted on a
shaft in the carburetor air horn
connected through linkage to a
thermostat mounted on the carbu-
retor throttle body. The thermo-
stat contains a bi-metal spring
and a vacuum actuated piston. A
choke rod connects the choke
valve to a fast idle cam on the
throttle body. A heat pipe con-
nects the choke housing to a heat
stove in the right exhaust mani-
fold. See Figure 3-46.
The heat stove in the exhaust
manifold heats the air which is
drawn through it and the heat
pipe into the choke housing. A
restriction in the choke housing
cover regulates the quantity of air
flowing into the choke housing to
heat the thermostat.
When the accelerator pedal is
depressed preparatory to starting
the engine, the throttle stop screw
is lifted clear of the fast idle cam
and the thermostat then closes the
choke valve. When the engine
starts, intake manifold vacuum
causes the piston to partially open
the choke valve against the spring
tension of the thermostat, thereby
admitting sufficient air to give a
satisfactory running mixture.
If the throttle is partially opened
while the running engine is cold,
the increased force of air flow
against the off-set choke valve
will open the valve against the
spring tension of the thermostat.
At the same time the choke valve
opens, the fast idle cam will also
drop to a new position, thereby
reducing the fast idle speed when
the throttle is again closed.
As the engine and exhaust mani-
fold warm up, warm air is drawn
through the heat pipe into the
choke housing by manifold vac-
uum. This warms the thermostat,
causing it to reduce its spring
tension on the choke valve in
proportion to the increase in tem-
perature. This, in turn, allows
the choke valve to be opened by
the combined forces of air veloc-
ity on the valve and vacuum on
the choke piston.
When the engine reaches normal
operating temperature, the choke
thermostat is heated to the point
CHOKE
VALVE
CHOKE
THERMOSTATIC
COIL /
CHOKE
HOUSING
CHOKE
PISTON
INTERMEDIATE
CHOKE SHAFT
a LEVER
Figure 3-46—Choke System