GMC 1976 ZEO 6083 Maintenance Manual page 348

6M- 6 ENGINE FUEL SYSTEM
bottom of the pump well insuring an adequate fuel
supply until the fuel starts to flow from the main
discharge nozzle .
Due to vacuum at the pump jets during high
speed operation, the pump discharge passage has
been vented to the top of the air horn, outside the
carburetor bores to balance the air pressure on the
fuel in the pump discharge passage with the fuel in
the float bowl . This prevents fuel from being pushed
out of the pump jets when the pump is not in use.
CHOKE SYSTEM (FIGURE 8)
The choke system consists of a choke valve, a
vacuum break diaphragm, a choke housing and coil
located on the side of the float bowl, fast idle cam,
connecting linkage and air valve lockout lever. The
thermostatic coil holds the choke valve closed when
the engine is cold . Opening the throttle valves allows
the choke to close and move the fast idle cam to the
high step . When the choke valve is closed, the air
valve lockout lever keeps the air valve closed .
During engine cranking, the choke valve is held
closed by the tension of the thermostatic coil . This
restricts air flow through the carburetor to provide
a richer starting mixture. When the engine starts,
manifold vacuum applied to the vacuum diaphragm
opens the choke valve to a point where the engine
will run without loading or stalling . The vacuum
diaphragm unit has an internal bleed check valve
which delays the diaphragm action a few seconds
before it becomes seated allowing the engine mani-
fold to be wetted and engine friction to decrease so
that when the vacuum break point is reached, the
engine will run without loading or stalling . When the
choke valve moves to the vacuum break position, the
fast idle cam follower will drop from the high step on
the fast idle cam to the next lower step when the
AIR VALVE
LOCK-OUT
AIR
LEVER
VALVE
CHOKE
ROD
FAST IDLE
CAM FOLLOWER
CHOKE
VALVE
"O" RING SEAL
PLASTIC RETAINER
OF
11
m
1
g111 ` ~/~,
ll~))~
CHOKE
COVER
THERMOSTATIC
COIL
INSIDE
CUP BAFFLE
AIR
BLEED
FILTER
SCREEN
BLEED CHECK
VALVE
CHECK VAL
VACUUM
BREAK
ROD
FAST IDLE
ADJUSTING
SCREW
DELAYED VACUUM
BREAK DIAPHRAGM
UNIT
A-1692
Figure 8-Choke System
throttle is opened . This gives the engine sufficient
fast idle speed and correct fuel mixture for running
until the engine begins to warm up and heat the
thermostatic coil in the choke housing. Engine
vacuum pulls heat from the manifold heat stove into
the choke housing and gradually relaxes choke coil
tension which allows the choke valve to continue
opening through inlet air pressure pushing on the off
set choke valve . Choke valve opening continues until
the thermostatic coil is completely relaxed, at which
point the choke valve is wide open and the engine is
thoroughly warm .
During the last few degrees of choke valve open-
ing, a tang on the choke lever contacts the secondary
air valve lockout lever and rotates the lever counter
clockwise so that the tang over the air valve will
move completely away from the valve, allowing the
air valves to open and operate.
The choke system is equipped with an unloader
mechanism which is designed to partially open the
choke valve, should the engine become loaded or
flooded. To unload the engine the accelerator pedal
must be depressed so that the throttle valves are held
wide open . A tang on a lever on the choke side of the
primary throttle shaft contacts the fast idle cam and
through the intermediate choke shaft forces the
choke valve slightly open . This allows extra air to
enter the carburetor bores and pass on into the en-
gine manifold and cylinders to lean out the fuel mix-
ture so that the engine will start.
CARBURETOR DIAGNOSIS
Before proceeding with carburetor diagnosis it
should be noted that proper carburetor operation is
dependent upon the following:
1. Fuel supply .
2. Linkage and emission control systems.
3. Engine compression .
4. Ignition system firing voltage.
5. Ignition spark timing .
6. Secure intake manifold .
7. Engine temperature.
8. Carburetor adjustments .
ANY PROBLEMS IN THE ABOVE AREAS
CAN CAUSE THE FOLLOWING :
1 . No start or hard starting - (hot or cold)