Pontiac FIREBIRD 1972 Service Manual page 696

6B-42 1972 PONTIAC SERVICE MANUAL
HlGH SPEED PA
MAlN VENTURI
MAlN NOZZLE
MAlN METERING
THROTTLE
VALVE
I
Fig. 6B-54 Main Metering System
the small venturi. Since the low pressure (high vacuum)
is now in the small venturi, fuel will flow in the following
manner:
Fuel from the float bowl passes through the main metering
jets (Fig. 6B-54) into the main well and rises in the main
well tubes. Air entering the main well through the main
well air bleeds is mixed with fuel through calibrated holes
in the main well tube. The mixture then moves up and out
of the discharge nozzle into a channel where more air is
added. The mixture then travels down the channel to the
small venturi where it is delivered to the air stream passing
through the venturi and then on into the intake manifold.
An additional fuel circuit has been provided which supple-
ments the main metering system of the carburetor unit. In
order to provide sufficient enrichment to the main meter-
ing system at higher air flows, two additional fuel feeds are
located in the air horn just above the choke valve. They
connect directly to the fuel in the float bowl, through
channels which lead directly into a tube that extends into
the fuel just above the main metering jets. At approxi-
mately 8 lbs. of air per minute and above, the extra fuel
enrichment is added to supplement the main metering
system.
With the addition of the pull over enrichment system,
leaner mixtures can be maintained during the part throttle
or cruising ranges and extra fuel supplied at higher air
flows to meet engine demands.
POWER ENRICHMENT SYSTEM
To achieve the proper mixtures required when more
power is desired or for extreme high speed driving, a
vacuum operated power piston in the air horn and a power
valve located in the bottom of the float bowl are used (Fig.
6B-55). Through a connecting vacuum passage from the
base of the carburetor to the power piston cylinder in the
air horn, the power piston is exposed to engine manifold
vacuum at all times.
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H l G H SPEED
PULLOVER C l R C U l l
Fig. 6B-55 Power Enrichment System
During idle and part throttle operation, the relatively high
vacuum holds the power piston up against spring tension
and the power valve remains closed. Increase in engine
load lowers the manifold vacuum. When it has dropped
sufficiently, the power piston spring overcomes the up-
ward vacuum flow and the power piston moves down-
ward, opening the power valve to allow additional fuel to
flow through calibrated restrictions into the main well.
As the engine load decreases, the resulting higher vacuum
overcomes the spring tension on the power piston and
raises the power piston, closing the power valve. It should
be noted that the power piston cavity in the carburetor air
horn is connected to the main air flow passage by a
vacuum relief passage. The purpose of this passage is to
prevent the transfer of vacuum acting on the piston from
also acting on the top of the fuel in the float bowl. Any
leakage of air past the upper grooves of the piston will be
compensated for by this relief passage and will not affect
carburetor metering.
PUMP SYSTEM
When the throttle valve is opened rapidly, air flow and
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PUMP
R E T U R N SPRING
Fig.
66-56
Pump System