Pontiac FIREBIRD 1972 Service Manual page 670

6B- 16 1972 PONTIAC SERVICE MANUAL
air is not needed, the bi-metal strip closes the valve and
operation returns to normal.
The compensator valve assembly is held i n place by the
dust cover over the valve chamber. A seal is used between
the compensator valve and float bowl casting.
In order to insure proper idle adjustment when the engine
is hot, the compensator valve must be closed. To check
this, plug the compensator inlet hole inside the air horn
bore (pencil can be used). If no drop in engine rpm is noted
on a tachometer, the valve is closed; if the valve is open,
leave plug in hole when adjusting idle or cool engine down
to a point where the valve automatically closes for proper
idle adjustment.
CAUTION: Always remove plug used in inlet
hole after completing idle adjostment, other-
wise, the compensator will not operate.
MAlN METERING SYSTEM
The main metering system (Fig. 6B- 16) supplies fuel to the
engine from off-idle to wide open throttle operation. It
feeds fuel at all times when air flow through the venturi
is great enough to maintain efficient fuel flow from the
main discharge nozzle. The triple venturi stack-up used in
the Monojet carburetor is ultrasensitive to air flow, which
results in a finer and more stable metering control from
light to heavy engine loads.
The main metering system consists of a main metering jet,
mechanical and vacuum operated metering rod, main fuel
well, main well air bleeds, fuel discharge nozzle and triple
venturi.
The main metering system operates in the following man-
ner:
MAlN WFLL AIR BLEED
WER PISTON
METERING ROD
POWER PISTON
BOOST VENTUR
I A I N VENTURI
MAlN METERIN(
CALIBRATION
LOWER IDLE
/
POWER PISTON VACUUM
THROTTLE VALVE
Fig. 6B-16 Main Metering System
As the throttle valve is opened beyond the off-idle range,
allowing more air to enter the engine manifold, air velocity
increases in the carburetor venturi. This causes a drop in
pressure in the main venturi which is increased many
times in the double boost venturi. Since the lower pressure
(vacuum) is now in the smallest venturi, fuel flows from
the main discharge nozzles in the following manner:
Fuel in the float bowl passes between the tapered metering
rod and the main metering jet where it is metered and
flows on into the main fuel well. In the main well the fuel
is mixed with air from the air bleed at the top of the well
and another air bleed which leads into the main well from
the discharge nozzle cavity. After the fuel in the main well
is mixed with air from the air bleeds it then passes up the
discharge nozzle where it sprays into the small boost ven-
turi. At the boost venturi, the fuel mixture then combines
with air entering the engine through the carburetor bore
to provide the correct air/fuel mixtures to the engine for
efficient combustion.
Fuel flow to the main discharge nozzle is controlled by a
tapered metering rod which is actuated by linkage con-
nected directly to the throttle shaft. As the throttle valve
is opened from idle position, the tapered metering rod is
gradually raised out of the main metering jet orifice. Fuel
flow from the main discharge nozzle is controlled by
throttle opening and the depth of the metering rod in the
main metering jet orifice. With the fuel metering mechani-
cally controlled by the throttle valve angle, it is possible
to maintain very accurate mixture ratios throughout part
throttle to wide open throttle operation. An initial meter-
ing rod adjustment is required to set the depth of the rod
in the main metering jet.
CAUTION: It should be noted here that there is
a supplementary fuel feed passage in the bottom
of the float bowl adjacent to the main metering
jet. Fuel is picked up fiom the float bowl and
passes through a calibrated hole, past a calibra-
tion screw and on into the same fuel passage
which leads fiom the main metering jet to the
m a i n fuel well. The purpose of the adjostable
fuel feed is to allow the factory to refine part
throttle calibration to meet very accurate air/f-
uel mixture ratios. This adjustment is made us-
ing very sensitive instrumentation and the
screw should not be tampered with or it will
require complete float bowl or unit replace-
ment.
POWER ENRICHMENT SYSTEM
The vacuum operated power enrichment system (Fig. 6B-
17) is use'd to slightly enrichen mixture ratios during mod-
erate to heavy loads during acceleration. The necessary
enrichment is obtained by movement of a spring loaded
vacuum piston which senses changes in manifold vacuum.
The amount of enrichment is controlled by the clearance
between the groove in the power piston and the diameter
of the power piston drive rod.