Pontiac FIREBIRD 1972 Service Manual page 714

6B-60 1972 PONTIAC SERVICE MANUAL
' 1
BOWL
Fig.
68-76 Float System
cleaner. The large internal dome vent will provide a com-
pletely internally balanced carburetor and yet allow the
elimination of the external idle vent valve. Any fuel vapors
which may form in the float chamber during hot engine
operation will be condensed in the dome vent and allowed
to drain back into the fuel bowl.
A completely internally balanced carburetor can be main-
tained with this feature because the pressure acting upon
the fuel in the float bowl will be balanced with the air
passing through the air cleaner.
IDLE SYSTEM
The idle system (Fig. 6B-77) is only used in the two pri-
mary bores of the carburetor. Each bore has a separate idle
system. They consist of idle tubes, idle passages, idle air
bleeds, idle channel restrictions, idle mixture needles and
idle discharge holes.
During curb idle, the primary throttle valves are held
slightly open by the throttle stop screw to give the engine
the desired idle speed. Since the engine requires very little
air for idle and low speeds, the idle discharge holes below
the throttle valves are exposed directly to engine manifold
vacuum. With the idle discharge holes in a very low pres-
sure area and the fuel in the float bowl vented to atmos-
phere (high pressure), the idle system operates as follows:
Engine manifold vacuum at the idle discharge ports causes
fuel to flow from the float bowl through the primary main
metering jets into the main fuel wells. It passes from the
main fuel well into the idle passages where it is picked up
by the idle tubes. The fuel is metered at the tip of the idle
tubes and passes up through the idle tubes. The fuel is
mixed with air at the top of each idle tube through an idle
air bleed. The fuel mixture then crosses over to the idle
down channels where it passes through a calibrated idle
channel restriction.
It then passes down the idle channel past the lower idle
air bleed holes and off-idle discharge ports, just above the
primary throttle valves, where it is mixed with more air.
The air/fuel mixture then moves down to the idle needle
discharge holes, where it enters the carburetor bores and
finally mixes with air passing around the slightly open
throttle valve. It then enters the intake manifold and is
conducted to the engine cylinders as a combustible mix-
ture.