Pontiac FIREBIRD 1972 Service Manual page 693

ENGINE FUEL 6B-39
vacuum is applied through the check valve to the
vacuum break diaphragm, causing the vacuum break
diaphragm to move slowly inward. This gives suffi-
cient time to overcome engine .friction and allow the
engine manifold to be wetted to prevent a lean stall.
A choke closing assist spring has been added to the
vacuum break diaphragm plunger stem. The spring
assists in closing the choke valve, along with tension
from the remote choke thermostatic coil, for im-
porved cold starting. The choke closing assist spring
only exerts pressure on the vacuum break link to
assist in closing the choke valve during engine start-
ing.
When the engine starts and the choke vacuum break
diaphragm seats, the closing spring retainer hits a
stop on the plunger stem and no longer exerts pres-
sure on the vacuum break link.
The 1972 model 2GV for Pontiac is similar in operation
to the 1971 models, except for the following design
changes:
1. A new throttle lever is used to meet the 1972 engine
design change requirements.
2. The pull over enrichment circuit is used on all model
2GV applications to provide the desired enrichment
at higher engine speeds.
3. A variable choke vacuum break unit is used which
has a tension spring mounted in the plunger head that
works against the tension of the thermostatic coil.
Incorporated in the variable vacuum break unit is a
bleed check valve which delays opening of the choke
valve to the vacuum break position a few seconds,
until the engine will run at a slightly leaner mixture.
4. The carburetor has been completely recalibrated to
meet the 1972 engine requirements.
The Model 2GV carburetor is of the side bowl design. The
carburetor float bowl is located forward of the main bores
of the carburetor. The carburetor is compact in design in
that all fuel metering is centrally located. The carburetor
uses a calibrated cluster design in which is placed the main
well tubes, idle tubes, mixture passages, air bleeds and
pump jets. The cluster can easily be removed for cleaning
and inspection purposes. The venturi cluster fits on a flat
portion of the carburetor bowl in front of the main venturi
with a gasket underneath. The idle and main well tubes are
installed in the cluster body by means of a precision press
fit and, therefore, cannot be serviced separately. The main
nozzles and idle tubes are suspended in the fuel in the
main wells of the float bowl.
The main metering jets are of a fixed type. Metering cali-
bration is accomplished through a system of calibrated air
bleeds, which give the correct air/fuel mixtures through-
out all operational ranges.
The model 2GV carburetor employs tHe use of a vacuum
operated power system for extra power when needed.
Power mixtures are controlled by a drop in engine mani-
fold vacuum, regardless of the degree qf throttle valve
opening. Thereby, additional fuel can be supplied for
power enrichment according to engine demands.
The pump system has a vented pump plunger. The pump
cup is of a synthetic material with an expander sprillg
located between the cup and the plunger head. This en-
sures proper seating of the pump plunger cup at all times.
The pump well is vented through the pump plunger head
by channels between the rubber cup and the plastic pump
plunger head, which leads from the pump well through
the center of the pump cup bore, up past the upper sealing
surface and outward into the float bowl. By venting the
pump plunger, any vapors which form in the pump well
are vented to the float bowl during hot engine operation.
This ensures that the pump well and passages will be
primed with solid fuel at all times, thereby, improving
accelerator pump action.
The carburetor is internally vented through a hole which
leads from beneath the carburetor air cleaner, inside the
air horn bore, to the fuel in the top of the float bowl.
Fig. 68-5 1 2GV Carburetor Identification Location