Pontiac FIREBIRD 1972 Service Manual page 669

ENGINE FUEL 6B- 1 5
TOP AIR BLEED
-\
IDLE CHANNEL
RESTRICTION
METERING JET
OMPENSATOR
IDLE DISCHARGE HOLE NEEDLE
OFF-IDLE
OPERATION
Fig. 68-1 5 Idle System
below the throttle valve. With the idle discharge hole in
a very low pressure area and the fuel in the float bowl
vented to atmosphere, fuel flows through the idle system
as follows:
Atmospheric pressure forces fuel from the float bowl
down through the main metering jet into the main fuel
well where it is picked up and metered at the lower tip of
the idle tube. It passes up the idle tube and is mixed with
air at the top of the idle channel through the idle air bleed
hole. The air/fuel mixture passes over through the cross
channel and then downward through the calibrated idle
channel restriction where it is further metered. The mix-
ture continues down the idle passage past the lower idle
air bleed hole and off-idle discharge port just above the
throttle valve, where it is again mixed with air. The air/f-
uel mixture then moves downward past the idle mixture
needle and out through the idle discharge hole into the
carburetor bore. Here it mixes with the air passing around
the slightly open throttle valve and then continues
through the intake manifold into the engine cylinders as
a combustible mixture.
OFF-IDLE OPERATION
As the throttle valve is opened from curb idle to increase
engine speed, additional fuel is needed to combine with the
extra air entering the engine. This is accomplished by the
slotted off-idle port. As the throttle valve is opened, it
passes the off-idle port, gradually exposing it to high
vacuum below the throttle valve. The additional fuel from
the off-idle port mixes with the increased air flow past the
opening throttle valve to meet increased engine air and
fuel demands.
Further opening of the throttle valve causes increased air
flow through the carburetor bore, which causes sufficient
pressure drop in the multiple venturi to start fuel delivery
from the main discharge nozzle. The off-idle port fuel
discharge does not cease at this transfer point but rather
diminishes as fuel flow from the main discharge nozzle
increases. In this way, the systems are so designed that
they combine to produce a smooth fuel flow at all engine
speeds.
The lower idle air bleed is used strictly as an air bleed
during idle operation. It supplies additional air to the idle
circuit for improved atomization and fuel control at low
engine speeds.
The same air bleed is used as an additional fuel feed at
higher engine speeds to supplement main discharge nozzle
delivery during operation of the main metering system.
The timed spark port has two tubes which supply vacuum
during the off-idle and part throttle operation of the car-
buretor. One tube is connected by a rubber hose to the
C.E.C. valve to supply spark vacuum advance during high
gear operation of the vehicle. The other tube leads to the
purge valve on the vapor canister to provide a means of
pulling fuel vapors form the canister during periods of
higher air flow through the carburetor bore. A limited
amount of canister purge is also provided by a separate
tube which leads from the canister to the PCV valve hose
connection.
HOT IDLE COMPENSATOR
The hot idle compensator (Fig. 6B-14), with automatic
transmission only, is located in a chamber on the float
bowl casting, adjacent to the carburetor bore, on the throt-
tle lever side of the carburetor. Its purpose is to offset
enrichening effects caused by changes in air density and
fuel vapors generated during hot engine operation.
The compensator consists of a thermostatically controlled
valve, a bi-metal strip which is heat sensitive, a valve
holder and bracket. The valve closes off an air channel
which leads from a hole inside the air horn to a point
below the throttle valve where it exits into the throttle
body bore.
Normally, the compensator valve is held closed by tension
of the bi-metal strip and engine vacuum. During extreme
hot engine operation, excessive fuel vapors in the carbu-
retor can enter the engine manifold causing richer than
normally required mixtures. This can result in rough en-
gine idle and stalling. At a pre-determined temperature,
.when extra air is needed to offset the enrichening effects
of fuel vapors, the bi-metal strip bends and unseats the
compensator valve, uncovering the air channel leading
from the compensator valve chamber to the throttle body
bore. This allows enough air to be drawn into the engine
manifold to offset the richer mixtures and maintain a
smooth engine idle. When the engine cools and the extra