Pontiac FIREBIRD 1972 Service Manual page 683

ENGINE FUEL 6B-29
At approximately 20 MPH, the largest or economy step
of the metering rod extends into the jet, thereby giving the
smallest possible metering area. As the throttle valve is
opened for higher speed or greater power, the metering
rod is raised so that a tapered sectibn and later the smallest
or power step provides increased metering area between
rod and jet. At top speed, the smallest or power step is in
the jet.
Engines operated at part throttle on level road use a mix-
ture of maximum leanness. The mixture for greatest
power and acceleration is somewhat richer and is fur-
nished by the power and accelerating systems described
below.
OPERATION OF WGD POWER SYSTEM
For maximum power at any speed for all operation above
approximately 75 MPH, a richer mixture is required than
that necessary for normal throttle opening. The richer
mixture is supplied through the high speed systems of
both barrels of carburetor by means of the power system.
The power system consists of a vacuum piston located in
a cylinder connected to manifold vacuum, a spring which
tends to push the piston upward against manifold vacuum
and a vacumeter piston link attached to the piston and
supporting the two metering rods (Fig. 6B-35).
Under part throttle operation, manifold vacuum is suffi-
cient to hold the piston and link down against the tension
of the spring, so that the link is held against the tongue of
the metering rod arm. The metering rods are then raised
and lowered mechanically as the throttle valve is opened
and closed. When the throttle valve is opened to a point
where additional fuel is required for satisfactory operat-
ion, manifold vacuum decreases sufficiently so that the
piston spring moves the piston, link and metering rods
upward to the proper metering rod step position to give
the required richer mixture, independently of throttle
opening. As soon as the demand is passed, manifold
vacuum moves the piston link down against the metering
rod arm so that the metering rods are controlled mechani-
cally again.
OPERATION OF WGD ACCELERATING SYSTEM
The accelerating system supplies the extra quantity of fuel
which is needed momentarily for smooth and rapid accel-
eration when the throttle valve is suddenly opened.
A pump plunger, operating in a cylinder cast into the float
chamber, is mechanically operated from the throttle valve
shaft by means of the throttle shaft arm, throttle connec-
tor rod, pump operating arm and countershaft assembly,
pump arm and pump arm link. The pump circuit contains
intake and discharge check valves and discharge passage
leading to a pump jet in each barrel of carburetor (Fig.
6B-36).
RING
PIST(
AND SPRING
I
PASSAGE TO
4
K ~ ~ ~
p p p - - p p -
-
Fig. 6B-35 Power System
When the throttle is closed, the pump plunger moves up
and draws a supply of fuel from the float chamber past the
intake ball into the pump cylinder. When the throttle is
opened, the pump plunger on its downward stroke exerts
pressure on the fuel which presses the intake ball against
its seat, raises the check needle off its seat and discharges
a metered quantity of fuel through the pump jets into each
barrel of carburetor. This occurs only momentarily during
the accelerating period. The pump plunger spring pro-
vides a follow-up action so that the fuel discharge carries
out over a brief period of time (Fig. 6B-36).
-
r
Fig. 6B-36 Accelerating System