Cadillac 54-6219 1954 Shop Manual page 242

12-il
ENGINE FUEL AND EXHAUST
pump jets is vented by a cross passage to the
carburetor bowl above the fuel level. This allows
air to be drawn off instead of fuel. Actually, the
fuel discharge orifices are the inner pump jets, as
shown in the inset of Fig. 12-12. The fuel stream
never touches the walls of the outer jets which
shield the fuel from the airstream and which are
vented to the bowl.
Choke System
The climatic control, or automatic choke, pro
vides a richer mixture as required for quick cold
engine starting and smooth operation during the
warm-up period, as illustrated in Fig. 12-13.
Choking is necessary only on the primary side
of the carburetor, since the secondary throttle
valves are locked in the closed position whenever
the choke is partially on. In addition, an extra
lockout pawl keeps the secondary lockout linkage
engaged until the primary throttle is returned to
idle position for the first time after the warm-up
period is over.
When the engine is cold, tension of the thermo
static coil spring holds the choke valve closed.
When the engine is started, air velocity against
the off-set choke valve causes the valve to open
slightly against the thermostatic spring tension.
Intake manifold vacuum applied to the choke piston
also tends to pull the choke valve open. The choke
valve assumes a position where tension of the
thermostatic
spring is balanced by the pull of
vacuum on the piston and force of air velocity
on the offset valve.
When the, engine starts, slots located in the sides
of the choke piston cylinder are uncovered allowing
intake manifold vacuum to draw warm air through
the hot air tube, from the hot air stove located on
the exhaust manifold, through the climatic control
housing. The flow of warm air in turn heats the
thermostatic spring and causes it to lose some of
its tension. The thermostatic spring loses
its
tension gradually until the choke valve reaches full
open position.
If the engine is accelerated during the warm-up
period, the corresponding drop in manifold vacuum
allows the thermostatic
spring to momentarily
close the choke, providing a richer mixture. The
choke modifying linkage between the thermostatic
spring
and throttle shaft relieves thermostatic
spring tension during cold engine acceleration.
During the warm-up period it is necessary to
provide a fast idle speed to prevent engine stalling.
This is accomplished by a fast idle cam which is
rotated by a connector rod attached to the choke
shaft. The fast idle cam prevents the primary
throttle valves from returning to a normal, warm
engine idle position while the climatic control is
in Operation.
If, during the starting period, the engine is
flooded, it is necessary to hold the choke open
swfficiently and remove the excessive fuel in the
intake manifold. This may be accomplished by
depressing the accelerator pedal to the floor
mat and engaging the starter. The projection on
the throttle lever unloader will rotate the fast
idle cam and in turn partially open the choke
valve.
Idle Speed-Up Control
Cars equipped with the Cadillac Air Condi
tioner have a vacuum powered, solenoid operated,
Idle Speed-Up Control attached to the carbure
tor. This device is designed to increase the en
gine idle RPM from 400 to 900, when the selector
is in the Neutral "N" position and the Air Condi
tioner switch is "On", to provide adequate cooling
and guard against possible overheating of the
engine during parking.
Fig. 12-12 Accelerator Pump System
Fig. 12-13 Choke System