Buick LeSabre 1965 Service Manual page 111

ENGINE FUEL AND EXHAUST
2-BARREL ROCHESTER 3 - 3 5
VACUUM PORT
CHOKE PISTON
THERMOSTATIC
COIL
CHOKE
HOUSING
VACUUM PASSAGE
Figure 3-32—Choke System
The choke thermostatic coil is
calibrated to hold the choke valve
closed when the engine is cold.
When the engine is started, air
velocity against the offset choke
valve causes the valve to open
slightly against the torque of the
thermostatic coil. In addition, in-
take manifold vacuum applied to
the choke piston through the vac-
uum passage tends to open the
choke valve. Vacuum pull on the
choke piston is offset by the ten-
sion of the thermostatic coil. As
the engine warms up, heated air
is drawn into the choke housing
through the choke heat tube
through a passage in the choke
housing. As the engine tempera-
ture i n c r e a s e s , it causes the
thermostatic coil to relax its ten-
sion, which together with vacuum
pull on the choke piston and air
flow against the offset choke valve
causes the choke valve to open
gradually until the e n g i n e is
thoroughly warmed up, at which
point the choke valve is fully
opened.
Automatic choke failure due to
build-up of dust or other foreign
material in the choke housing is
a common service item. This
dirt is trapped from the air which
is continually passing through the
choke whenever the engine is
running.
In past models, air for the choke
was taken in at the rear end of a
pipe passing through the exhaust
manifold passage, then up to the
choke housing through an insu-
lated pipe. This part of the choke
heat system remains the same;
however, all Buicks now have a
clean air pipe which conducts fil-
tered air from the carburetor air
horn to the intake end of the ex-
haust manifold pipe. Since nothing
but filtered air passes through
the choke housing with this new
system, the action of the auto-
matic choke remains trouble-free
for a much longer period of use.
A mechanical choke unloader is
incorporated to open the choke
valve slightly when the engine is
cold. The choke unloader provides
a means for opening the choke
valve to correct any loading con-
dition encountered d u r i n g cold
starting.
To prevent stalling during the
warm-up period, it is necessary
to run the engine at a slightly
higher idle speed than for a warm
engine. This is accomplished by
steps on the fast idle cam. The
fast idle cam is in turn linked to
the choke valve shaft by the choke
rod, choke trip lever and choke
lever and collar assembly. This
holds the throttle valves open suf-
ficiently during the warm-up
period to give increased idle RPM
until the choke valve moves to the
fully open position and the engine
is thoroughly warmed up.
3 - 1 5 DISASSEMBLY,
CLEANING AND
INSPECTION OF THE
ROCHESTER 2-BARREL
CARBURETOR
a. Choke Disassembly
and Removal of Air Horn
1. Mount carburetor on a proper
mounting fixture such as J-5923.
2. Remove three choke cover at-
taching screws and retainers.
Remove choke cover assembly,
gasket and insulator baffle inside
the choke housing.
3. Remove choke piston and lever
assembly from the end of the
choke shaft in the choke housing
by removing retaining screw in
the end of the choke shaft. Rotate
choke piston lever to remove
choke piston from bore in choke
housing. Choke piston can now
be removed from the lever by
shaking piston pin into palm of
hand.
4. R e m o v e two Phillips choke
housing attaching screws, then
remove the choke housing and
gasket from the air horn.
5. Remove pump rod by removing
upper and lower retaining clips.
6. Remove fast idle cam attach-
ing screw. Then remove fast idle
cam and rod assembly by rotating
until lug on upper end of choke
rod passes through slot in the
upper choke lever and collar as-
sembly. The lower end of choke
rod can be removed from fast
idle cam in the same manner.
7. R e m o v e air horn attaching
screws and carefully remove air
horn assembly from float bowl
by lifting gently upward.
b. Disassembly of Air Horn
1. Place air horn assembly in-
verted on bench. Remove float
hinge pin and lift float assembly
from cover. Remove float needle
from the float arm. Remove float
needle seat, fibre gasket and
needle seat screen. See Figure
3-33.
2. Remove power piston by de-
pressing shaft and allowing spring
to snap repeatedly, thus forcing
the power piston retaining washer
from casting.
CHOKE } ^ * l
VALVE — ' ^ * « s *
r - FLOAT
- P O W E R
PISTON
fte
L PUMP
PLUNGER
Figure 3-33—Air Horn Parts