Pontiac FIREBIRD 1972 Service Manual page 721

ENGINE FUEL
6B-67
AIR
AIR VALVE LOCK-OUT LEVER
COLD ENRICHMENT
FUEL CHANNEL
AIR VALVE
RESTRICTION
FAST IDLE ADJUSTING
HERMOSTATIC
HOKE COIL & ROD
Fig. 68-82 Choke Svstern
When the choke valve is closed, the air valve lock-out
lever is weighted so that a tang on the lever catches the
upper edge of the air valve and keeps the air valve closed.
During engine cranking, the choke valve is held closed by
the tension of the thermostatic coil. Two fuel feed holes,
located just beneath the choke valve, supply added fuel for
cold enrichment during cranking. The additional fuel is
supplied from tubes which lead from the bottom of the
float bowl to the two fuel feed holes in the air horn bores.
When the engine starts and is running, manifold vacuum
is applied to the vacuum break diaphragm unit on the flat
bowl and slowly opens the choke valve to a point where
the engine will run without loading or stalling. A bleed
check valve, located in the vacuum diaphragm unit, delays
choke valve opening to allow the manifold to be wetted
and prevent stalling due to leanness.
As the choke valve is opened to the vacuum break posi-
tion, the cold enrichment feed holes, no longer in a low
pressure area, cease to feed fuel.
Included in the vacuum break unit is a spring loaded
plunger. The purpose of the spring is to off-set choke
thermostatic coil tension and balance the greater opening
of the choke valve with tension of the choke coil. This
enables further refinement because the coil which senses
engine and ambient temperatures will allow the choke
valve to open gradually against spring tension in the dia-
phragm plunger head. In other words, in very cold tem-
peratures the extra tension created by the thermostatic
coil will overcome the tension of the diaphragm plunger
spring and provide less choke valve opening with the re-
sults of slightly richer mixture. In warmer temperatures
the thermostatic coil will have less tension and conse-
quently will not press the spring as much, thereby, giving
a greater choke valve opening for slightly leaner mixtures.
As the choke valve is opening, the fast idle cam follower
on the end of the primary throttle shaft will drop from the
highest step on the fast idle cam to the second step when
the throttle valve is opened. This gives the engine suffi-
cient fast idle and correct fuel mixtures for running until
the engine begins to warm up and heat the thermostatic
coil. As the thermostatic coil on the engine manifold
becomes heated, it relaxes its tension and allows the choke
valve to open further because of intake air pushing on the
off-set choke valve. Choke valve opening continues until