Buick LeSabre 1965 Service Manual page 129

ENGINE FUEL AND EXHAUST
4-BARREL CARTER 3 - 5 3
STEP-UP PISTON AND SPRING
VENTS TO
STEP-UP
PISTONS
HIGH-SPEED AIR
BLEED TUBES
Figure 3-69—Power Circuit
hold the piston and rod down
against the tension of the spring,
so that the large diameter of the
rod is in the metering jet for
economy. When the throttle valve
is opened to a point where addi-
tional fuel is required for satis-
factory o p e r a t i o n , m a n i f o l d
vacuum decreases sufficiently so
that the piston spring moves the
piston and rod upward to the small
rod diameter to give the required
richer mixture for power. As
soon as the demand is passed
manifold vacuum again moves the
piston and rod down.
f. Operation of
Accelerating System
The accelerating pump circuit,
located in the primary side, pro-
vides a measured amount of fuel
necessary to insure smooth en-
gine operation on acceleration at
lower car speeds.
When the throttle is closed, the
pump plunger moves upward in
its cylinder and fuel is drawn into
the pump cylinder through the in-
take check. The discharge check
is seated at this time to prevent
air from being drawn into the cyl-
inder. When the throttle is opened,
the pump plunger moves down-
ward forcing fuel out through the
discharge passage, past the dis-
charge check, and out of the pump
jets. When the plunger moves
downward, the intake check is
closed, preventing fuel from being
forced back into the bowl. See
Figure 3-70.
At higher car speeds, pump dis-
charge is no longer necessary to
PUMP SET
DISCHARGE
CHECK
Figure 3-70—Pump Circuit
insure smooth acceleration. When
the throttle valves are opened a
predetermined amount, the pump
plunger bottoms in the cylinder
e l i m i n a t i n g f u r t h e r pump
discharge.
Be sure the pump plunger cup
is in good condition and the intake
and discharge checks and pump
jet are free of lint, gum or other
foreign matter. The pump intake
check is a one piece assembly
located in an angular passage at
the base of the pump assembly.
The intake check is serviced only
as a complete assembly.
g. Operation of
Choke System
The choke circuit, located in the
primary side, provides the cor-
rect mixture necessary for quick
cold engine starting and during
engine warm-up.
When the engine is cold, tension
of the thermostatic coil holds the
choke valve closed. When the en-
gine is started, a i r v e l o c i t y
against the offset choke valve
causes the valve to open slightly
against the thermostatic coil ten-
sion. Intake manifold vacuum ap-
plied to the choke piston also
tends to pull the choke valve open.
The choke valve assumes a posi-
tion, where tension of the thermo-
static coil is balanced, by the pull
of vacuum on the piston, and force
of air velocity on the offset valve.
When the engine starts, slots lo-
cated in the sides of the choke
piston cylinder are uncovered,
allowing intake manifold vacuum
to draw warm air through the
climatic control housing. This air
is h e a t e d in a t u b e running
t h r o u g h the exhaust manifold.
Clean, filtered air is used to op-
erate the choke thermostatic coil.
Air is taken from the air horn of
the carburetor, down a pipe to
the right exhaust manifold. The
flow of warm air heats the ther-
mostatic coil and causes it to
lose some of its tension. The