Operating Systems - Pontiac FIREBIRD 1972 Service Manual

ENGINE FUEL 6B-59
The large internal vent vents float bowl vapors to the air
cleaner base.
To provide additional air capacity through the primary
bores, the second boost venturi has been removed and the
small boost venturi skirt lengthened (455 H.O. ONLY).
The choke vacuum break diaphragm has a varibale feature
which includes a compression spring on the plunger stem.
The internal check valve is replaced by an internal bleed
restriction, which retards choke valve opening.
The carburetor part number is stamped on a vertical sec-
tion of the float bowl, near the secondary throttle lever
(Fig. 6B-75.) When replacing the float bowl assembly,
follow the manufacturer's instructions contained in the
service package so that the part number can be transferred
to the new float bowl.
The primary side of the carburetor has six systems of
operation. They are float, idle, main metering, power,
pump and choke. The secondary side has one metering
system which supplements the primary main metering
system and receives fuel from a common float chamber.
SHIFT
.
IDENT.
' 7
.ANT CODE
YEAR
D A Y OF YEAR
Fig. 68-75 4MV Carburetor Identification Location
OPERATING SYSTEMS
FLOAT SYSTEM
The Quadrajet carburetor has a centrally located float
chamber (Fig. 6B-76) with a single pontoon float and float
needle valve. The fuel bowl is centered between the pri-
mary bores and is adjacent to the secondary bores. This
type design assures adequate fuel supply to all carburetor
bores, which gives excellent performance with respect to
car inclination or severity of turns.
The float pontoon is solid and is made of a closed cell
plastic material. It is lighter than a brass pontoon, which
results in added buoyancy and allows the use of a smaller
float to maintain constant fuel levels.
A pleated paper fuel inlet filter is used with a pressure
relief spring. The relief spring allows fuel pump pressure
to force the filter off its seat if for any reason it should
become clogged and prevent fuel flow to the carburetor.
A guide is cast into the filter housing to prevent the possi-
bility of the filter being installed in the reverse position.
The float system consists of a float chamber, plastic float
pontoon assembly, float hinge pin and retainer combina-
tion, a float needle valve and seat and a float valve pull
clip. The float system operates as follows:
Fuel from the engine fuel pump enters the carburetor fuel
inlet passage. It passes through the filter element and
needle seat.
As incoming fuel fills the float bowl to the prescribed fuel
level, the float pontoon rises and forces the fuel inlet valve
closed, shutting off all fuel flow. As fuel is used from the
float chamber, the float drops and allows more incoming
fuel to enter the float bowl until the correct fuel level is
reached. This cycle continues, constantly maintaining a
positive fuel level in the float bowl.
A float needle pull clip, fastened to the float valve, hooks
over the center of the float arm. Its purpose is to assist in
lifting the float valve off its seat.
The fuel enters the float chamber at the top to prevent
incoming fuel Vapors from mixing with solid fuel in the
bottom of the float bowl and disrupting good carburetor
metering. A plastic filler block is located in the top of the
float chamber in the area just above the float valve. This
block prevents fuel slosh on severe brake applications.
This maintains a more constant fuel level during this type
maneuver, to prevent stalling.
The carburetor float chamber is internally vented through
a large dome vent which leads to the base of the air