Pontiac FIREBIRD 1972 Service Manual page 695

ENGINE FUEL 6B-4 1
IDLE SYSTEM
During engine idle operation, air flow through the carbu-
retor venturi is very low and is not great enough to cause
fuel to flow from the main dischaige nozzles. Therefore,
the idle system (Fig. 68-53) is used to provide the proper
mixture ratios required during idle and low speed operat-
ion of the engine.
The idle system consists of the idle tubes, idle passage
idle air bleeds, idle mixture needles, off-idle discharg
ports and the idle mixture needle holes.
In the idle position, the throttle valves are slightly open,
allowing a small amount of air to pass between the wall
of the carburetor bore and the edge of the throttle valves.
Since there is not enough air flow for venturi action, the
fuel is made to flow by the application of vacuum (low
pressure) directly through the idle system to the fuel in the
carburetor bowl.
Fuel from the float bowl passes through each main meter-
ing jet into the main fuel wells where it is metered by the
orifice at the lower tip of the idle tubes. It then passes up
the idle tubes and is mixed with air at the top of the idle
tubes by calibrated idle air bleeds. The air/fuel mixture
then passes downward through a calibrated restriction in
the vertical passage, then past a second idle bleed just
below the restriction and on downward past the off-idle
discharge port to the idle mixture holes. The fuel mixture
passes out the idle needle hole where it combines with air
passing the slightly open throttle valves. The idle mixture
needle controls the amount of fuel mixture which enters
the carburetor bore at curb idle position of the throttle
valves.
The idle mixture is pre-set at the factory and capped so
that no adjustment will be required on the engine. If clean-
ing or mixture needle replacement is necessary during
service operations, an adjustment procedure and new li-
miter caps will be provided in the repair kit.
A fuel vapor collection canister is used in all model vehi-
cles. The purpose of the vapor canister is to store raw fuel
vapors from the fuel tank which is vented to it, as the tank
has no external vents.
In order to purge the canister of these raw fuel vapors, a
constant purge system and a variable purge system are
included in the carburetor throttle body. They operate as
follows: The constand bleed and variable bleed ports are
connected to a common tube located in the throttle body
casting between the two idle mixture needles. The tube
connects directly to the vapor canister through a hose.
During normal engine idle, the constant bleed purge is in
operation and is located below the throttle valve. This is
a very small bleed hole which leads into the carburetor
bore area and is used to pull vapors from the vapor canis-
ter during engine idle.
In order to provide additional purge to the vapor canister,
two extra purge ports, located adjacent to the off-idle
discharge slots, come in to operation during the off-idle
and part throttle operation of the engine. This provides
enough additional purge to the vapor canister for remov-
ing all vapors from it.
OFF-IDLE OPERATION
As the throttle valves are opened during acceleration,
more fuel is needed and is supplied by the off-idle dis-
charge port, which is gradually exposed to manifold
vacuum. This port supplies additional fuel mixture for the
increased engine speed requirements. Improved fuel con-
trol is achieved by an adjustable off-idle air bleed screw.
The adjustment screw regulates air passing from a sepa-
rate channel into the idle channel and adjustment is made
at the factory. The adjusting screw is then sealed because
this adjustment cannot be made in the field without ad-
versely affecting carburetor calibration.
The hot idle compensator is used on air conditioned, F
Series models (Auto. trans.) to offset enrichening effects
caused by excessive fuel vapors from fuel percolation, dur-
ing extreme hot engine operation. The compensator con-
sists of a thermostatically controlled valve mounted in the
area above the main venturi. The valve closes off an air
channel which leads from above the carburetor venturi to
a point below the throttle valves.
The compensator valve is operated by a bi-metal strip
-
which senses temperature. During extreme hot engine op-
eration, excessive fuel vapors entering the engine manifold
cause richer than normally required mixtures, resulting in
rough engine idle and stalling. At a certain pre-determined
temperature, when extra air is needed to offset the enri-
chening effects of fuel vapors, the bi-metal strip bends and
unseats a valve which uncovers the air channel leading
from the carburetor venturi to below the throttle valves.
At this time, just enough air is added to the engine to offset
the richer mixtures and maintain a smooth engine idle.
When the engine cools, the extra air is not needed, the
bi-metal strip closes the valve and operation returns to
normal mixtures.
In order to ensure proper idle adjustment, the valve
should always be closed when setting engine idle speed
and mixtures.
MAIN METERING SYSTEM
As the throttle valves continue to open, the edge of the
throttle valves are gradually moving away from the wall
of the carburetor bore, reducing the vacuum so that the
discharge of the fuel mixture at the idle needle hole and
off-idle port gradually diminishes.
With the increased throttle opening, there is increased
velocity in the venturi system. This causes a drop in pres-
sure in the large venturi which is increased many times in