Pontiac FIREBIRD 1972 Service Manual page 715

ENGINE FUEL
6B-6 1
PURGE TUBE- CONSTANT BLEED
DISCHARGE VALVE
(CANISTER PURGE) HOLE
Fig. 66-77 Idle System
In order to meet emission standards, the idle mixture
needles will be pre-adjusted at the factory and capped so
that they cannot be adjusted in the field. New limiter caps
will be provided in the overhaul kits, should it be neces-
sary to remove the idle mixture needles for overhaul or
cleaning purposes.
A fixed idle air by-pass system is used to supplement the
idle air passing by the slightly open throttle valves. The
purpose of the idle air by-pass system is to reduce the
amount of air going through the carburetor bores and still
maintain sufficient air for the correct idle speed. This
reduces the amount of air passing through the venturi
system to prevent the main fuel nozzles from feeding at
idle. The venturi system is very sensitive to air flow and
where large amounts of air are needed to maintain idle
speeds, the fixed idle air by-pass system is used.
Since the fuel tank is not normally vented to atmosphere,
all fuel vapors are collected in a vapor collection canister.
Purge ports for the canister are provided in the carburetor
throttle body. They consist of a variable purge plus the
constant bleed purge. The constant bleed purge is a small
orifice and is used during idle operation to provide some
removal of vapors from the collection canister.
The timed bleed purge is located above the throttle valves,
adjacent to the off-idle discharge ports. The timed bleed
purge holes provide sufficient purge to remove all vapors
that will be collected in the vapor canister. They are de-
signed to bleed constantly during the off-idle and part
throttle operation of the engine.
OFF-IDLE OPERATION
As the primary throttle valves are opened from curb idle
to increase engine speed, additional fuel is needed to com-
bine with the extra air entering the engine. This is accom-
plished by the slotted off-idle discharge ports (Fig. 6B-77).
As the primary throttle valves open, they pass by the
off-idle ports, gradually exposing them to high engine
vacuum below the throttle valves. The additional fuel ad-
ded from the off-idle ports mixes with the increasing air
flow past the opening throttle valves to meet increased
engine air and fuel demands.
Further opening of the throttle valves increases the air
velocity through the carburetor venturi sufficiently to
cause low pressure at the lower idle air bleeds. As a result,