GMC 1976 ZEO 6083 Maintenance Manual page 346

6M- 4 ENGINE FUEL SYSTEM
is to improve control of fuel during the main meter-
ing range.
Two calibrated holes, one in each primary bore
are located just above the choke valve and feed fuel
from the float bowl . During high carburetor air
flows, low pressure created in the air horn bore pulls .
fuel from the high speed fuel feeds, supplementing
fuel flow from the main metering system . The pull
over enrichment system begins to feed fuel at approx-
imately 8 lbs . of air per minute, and contines to feed
at higher engine speeds to, provide extra fuel neces-
sary for good engine performance.
POWER SYSTEM (FIGURE 5)
The power system in the Quadrajet carburetor
provides an extra rich mixture under heavy accelera-
tion 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 spring loaded power piston located in a
cylinder which is exposed to manifold vacuum . The
spring loaded power piston tends to push upward
against manifold vacuum .
On part throttle and cruising ranges, manifold
vacuum is 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. When engine load is increased to a point
where an extra rich mixture 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 allows 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 the engine
Figure 5-Power System
operation is returned to part throttle and cruising
ranges, increased manifold vacuum overcomes the
power piston spring and returns the larger portion of
the metering rod into the metering jet orifice giving
a leaner mixture.
When engine speed is increased to a point where
the primary side of the carburetor cannot supply
sufficient air and fuel requirements, the secondary
side of the carburetor is used .
The secondary throttle valves are actuated by a
connecting linkage to the primary throttle lever.
With the throttle valves opened, a low pressure
(vacuum) is created beneath the air valve. Atmos-
pheric pressure on top of the offset spring loaded air
valve forces the valve open allowing the required air
to flow through the throttle bores to meet engine
demands.
As the air valve opens, the upper edge passes the
accelerating well port causing a low pressure
(vacuum) at that point. Fuel starts flowing immedi
ately and continues to flow until the well is empty.
The fuel from the accelerating ports prevents a
momentary hesitation and provides an immediate
charge of fuel until air/fuel begins to flow from the
secondary discharge nozzles.
The secondary main discharge nozzles (one for
each secondary bore) are located just below the air
valve and above the secondary throttle valves in the
area of lowest pressure . As the air valve opens, it
rotates a plastic cam attached to the center of the
main air valve shaft. The cam lifts a lever attached
to the secondary main metering rods out of the sec-
ondary orifice plates . Fuel flows from the float bowl
through the secondary orifice plates into secondary
main wells, where it is mixed with air from the sec-
ondary mail well air bleed tubes. The air/fuel mix-
ture travels from the main wells to the secondary
discharge nozzles and is expelled into the secondary
bores. The air/fuel mixture is mixed with more air
traveling through the secondary bores and combined
with the air/fuel mixture delivered from the primary
bores enters the engine cylinders as a combustible
mixture.
As the secondary throttle valve is opened further,
the increase in air flow through the throttle bores
opens the air valve which rotates the eccentric cam
lifting the tapered secondary metering rods further
out of the metering orifice discs, increasing fuel flow
in direct portion to air passing through the secondary
throttle bores. By using this principle a correct
air/fuel ratio can be maintained throughout the
operation of the secondary side of the carburetor .
A baffle plate is used in each secondary bore
extending up and around the secondary fuel dis-