Pontiac FIREBIRD 1972 Service Manual page 718

6B-64 1972 PONTIAC SERVICE MANUAL
air bleeds. The purpose of the adjustable bleed is to refine
fuel mixtures to meet emission requirements. The bleed is
adjusted at the factory and should not be re-adjusted in the
field.
POWER ENRICHMENT SYSTEM
The power system (Fig. 6B-79) in the Quadrajet carbu-
retor provides extra mixture enrichment for power re-
quirements under heavy acceleration or high speed
operation. The richer mixture is supplied through the
main metering system in the primary and secondary sides
of the carburetor.
The power system located in the primary side consists of
a vacuum piston and spring, located in a cylinder con-
nected by a passage to intake manifold vacuum. The
spring located beneath the vacuum operated power piston
tends to push the piston upward against manifold vacuum.
On part throttle and cruising ranges, manifold vacuums
are sufficient to hold the power piston down against spring
tension so that the larger diameter of the metering rod tip
is held in the main metering jet orifice. Mixture enrich-
ment is not necessary at this point. However, as engine
load is increased to a point where extra mixture enrich-
ment is required, the spring tension overcomes the
vacuum pull on the power piston and the tapered primary
metering rod tip moves upward in the main metering jet
orifice. The smaller diameter of the metering rod tip al-
lows more fuel to pass through the main metering jet and
enrich the mixture flowing into the primary main wells
and out the main discharge nozzles.
When manifold vacuum rises and mixture enrichment is
no longer needed, the vacuum overcomes the power piston
spring tension and returns the larger portion of the meter-
ing rod into the metering jet orifice and back to normal
economy range. However, as the engine speed increases,
the primary side of the carburetor can no longer meet the
engine air and fuel requirements. To meet these demands,
the secondary side of the carburetor is used. The second-
ary section contains throttle valves, spring loaded air
valves, meteking orifice discs, secondary metering rods,
main fuel wells with air bleed tubes, fuel discharge nozzles
and accelerating wells and tubes. The secondary side oper-
ates as follows:
When the engine reaches a point where the primary bores
cannot meet engine air and fuel demands, the primary
throttle lever (through connecting linkage to the second-
ary throttle shaft lever) begins to open the secondary
throttle valves. As air flow through the secondary bores
creates a low pressure (vacuum) beneath the air valve,
atmospheric pressure on top of the air valve forces the air
valve open against spring tension. This allows the required
air for increased engine speed to flow past the air valve.
When the air valve begins to open, the upper edge of the
air valve passes the accelerating well port. As the valve
passes the port, it exposes the port to manifold vacuum.
The port will immediately start to feed fuel from the ac-
celerating wells.
The accelerating ports will prevent a momentary leanness
as the valve opens and the secondary nozzles begin to feed
fuel.
The secondary main discharge nozzles (one for each sec-
ondary bore) are located just below the air valve and above
the secondary throttle valves. They, being in the area of
lowest pressure, begin to feed fuel as follows:
When the air valve begins to open, it rotates a plastic cam
attached to the center of the main air valve shaft. The cam
pushes on a lever attached to the secondary main metering
rods. The cam pushes the lever upward, raising the meter-
ing rods out of the secondary orifice discs. Fuel flows from
the float chamber through the secondary orifice discs into
secondary main wells, where it is mixed with air from the
main well tubes. The air emulsified fuel mixture travels
from the main wells to the secondary discharge nozzles
and into the secondary bores. Here fuel mixture is mixed
with air traveling through the secondary bores to supple-
ment the air/fuel mixture delivered from the primary
bores and then goes on into the engine manifold and on
to the engine cylinders as a combustible mixture.
As the throttle valves are opened further and engine
speeds increase, increased air flow through the secondary
side of the carburetor opens the air valve to a greater
degree which, in turn, lifts the secondary metering rods
further out of the orifice discs. The metering rods are
tapered so that fuel flow through the secondary metering
orifice discs is directly proportional to air flow through
the secondary carburetor bores. In this manner, correct
air/fuel mixtures to the engine through the secondary
bores can be maintained by the depths of the metering
rods in the orifice discs.
There are three other features incorporated in the second-
ary
1.
2.
3.
metering system which are as ibllows:
The main well bleed tubes extend below the fuel level
in the main well. These actually bleed air into the fuel
in the well to quickly emulsify the fuel with air for
good atomization as it leaves the secondary discharge
nozzles.
The secondary metering -rods have a slot milled in the
side to ensure adequate fuel supply in the secondary
fuel wells. These are necessary because, when the air
valve is in the closed position, the secondary metering
rods are nearly seated against the secondary metering
orifice discs. During hot engine idle or hot soak, the
fuel could boil away out of the fuel well. The milled
slot allows enough fuel to by-pass the orifice disc to
keep the, main well filled during this period. This
ensures immediate fuel delivery from the secondary
fuel wells at all times.
A single bafle plate is used which extends into each
secondary carburetor bore and seats into the float