Pontiac FIREBIRD 1972 Service Manual page 671

ENGINE FUEL 6B-17
MAlN WELL AIR BLEED
POWER PISTON
IOOST VENTURI
M A l N VENTURI
M A l N METERING
POWER PISTON
LINKAGE
THROTTLE
/
\
'POWER PISTON VACUUM
VALVE LOWER IDLE AIR BLEED
Fig. 6 8 - 1 7 Power Enrichment System
During part throttle and cruising ranges, manifold
vacuum is sufficient to hold the power piston down
against spring tension. The upper part of the groove in the
power piston is held down against the top side of the drive
rod. This places the main metering rod lower in the jet for
maximum economy. On moderate to heavy accelerations,
manifold vacuum drops and the power piston spring
pushes the power piston up so that the lower edge of the
slot in the power piston strikes the bottom side of the drive
rod. This moves the tapered metering rod slightly upward
and out of the main metering jet, allowing more fuel to
flow through the jet, enrichening the fuel mixture slightly.
ACCELERATING P U M P SYSTEM
Extra fuel for smooth, quick acceleration is supplied by a
double spring loaded pump plunger (Fig. 6B-18). Rapid
opening of the throttle valve, when accelerating from low
speed, causes an immediate increase in air flow through
the carburetor bore. Since fuel is heavier than air, it re-
quires a short period of time for fuel flow through the
main discharge nozzle to catch up with the air flow. To
avoid leanness during this momentary lag in the fuel flow,
the accelerator pump furnishes a metered quantity of fuel
which is sprayed into the air stream. This mixes with the
increased air flow to supply the extra fuel needed until the
main discharge nozzles can feed the fuel required.
The accelerating pump is located at the side of the main
fuel bowl, adjacent to the venturi area. It consists of a
spring loaded pump plunger and pump return spring oper-
ating in a fuel well. The pump plunger is connected by
linkage directly to a lever on the throttle shaft.
"T" DISCHARGE
SPRING RETAINER
PUMP LEVER
PUMP PLUNGE1
& DURATION
PUMP RETURN
w
PUMP CUP OPERATlOl
Fig. 6 8 - 1 8 Pump System
When the pump plunger moves upward in the pump well,
as happens during throttle closing, fuel from the float bowl
enters the pump well through a slot in the side of the pump
well and flows past the synthetic pump cup seal into the
bottom of the pump well. The pump cup is a floating type
(the cup moves up and down on the pump plunger head).
-
When the pump plunger is moved upward, the flat on the
top of the cup unseats from the flat on the plunger head
and allows free movement of fuel through the inside of the
cup into the bottom of the pump well. This also vents any
vapors which may be in the bottom of the pump well so
that a solid charge of fuel can be maintained in the fuel
well beneath the plunger head.
When the throttle valve is opened, as happens during
acceleration, the connecting pump linkage forces the
pump plunger downward. The pump cup seats instantly
and fuel is forced through the pump discharge passage,
where it unseats the pump discharge check ball and passes
on through the passage to the pump jet located at the top
of the float bowl, where it sprays into the boost venturi
area.
The pump plunger is spring loaded; the upper duration
spring is balanced with the bottom pump return spring so
that a smooth sustained charge of fuel is delivered during
acceleration.
,
The pump discharge check ball prevents any pull over or
.
discharge of fuel from the pump jet when the accelerator
pump is not in operation. It also keeps the pump discharge
passage filled with fuel to prevent pump discharge lag.
The pump does not require adjustment in service as it is
preset during manufacture.