GMC 1976 ZEO 6083 Maintenance Manual page 347

Figure 6-Air Valve Dashpot
charge nozzles. Their purpose is to provide good fuel
distribution at lower air flows by preventing too
much fuel from going to the front of the engine .
The depth of the secondary metering rods in the
orifice plates in relation to the air valve position are
factory adjusted to meet the air/fuel requirements
for the specific engine model. No further adjustment
should be required .
AIR VALVE DASHPOT (FIGURE 6)
The secondary air valve is connected to the
vacuum break unit by a rod, to control the opening
rate of the air valve and prevent any fuel lag from the
secondary discharge nozzle .
Whenever manifold vacuum is above 5" to 6"
vacuum, the vacuum break diaphragm stem is seated
and pulls the rod to the end of the slot in the air valve
shaft lever, holding the air valve closed . However,
when the secondary valves are opened and manifold
vacuum drops below 5" to 6" vacuum, the spring in
the vacuum break unit will force the diaphragm off
its seat and allow the air valve to open . The rate of
movement of the air valve spring is controlled by a
restriction in 'he internal check valve in the vacuum
break unit .
ACCELERATING PUMP SYSTEM
(FIGURE 7)
During quick acceleration when the throttle
valves are opened rapidly, the sudden increase in air
flow passing the fuel discharge nozzles tends to leave
the fuel behind, which is heavier, causing a momen-
tary leanness . The accelerator pump provides an ad-
ditional charge of fuel during this time .
ENGINE FUEL SYSTEM
6M- 5
The accelerating pump system is located in the
primary side of the carburetor consisting of a spring
loaded pump plunger and pump return spring,
located in the pump bore . The pump plunger is ope-
rated by a pump lever on the air horn which is con-
nected directly to the throttle lever by a pump rod.
As the throttle is returned from an open to a
closed position, the pump return spring pushes the
pump plunger upward against the pump lever. As
the pump begins to move up, the discharge check ball
immediately seats in the pump discharge passage so
that no air will be drawn into the passage, which
could cause a hesitation . The pump well is always
filled with fuel from the float bowl through a slot in
the top of the pump well which is lower than the fuel
level. When the pump plunger moves up, the floating
pump cup unseats (moves down) from the flat sur-
face on the plunger head and allows free flowing 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 .
When the primary throttle valves are opened, the
connecting linkage forces the pump plunger down
instantly seating the pump cup against the plunger
forcing fuel through the discharge passage unseating
the discharge check ball . The fuel is then forced up
through a passage to the pump jets located in the air
horn and sprayed into the venturi area of each pri-
mary bore .
It should be noted that the pump plunger is
spring loaded . The top pump duration spring is cali-
brated so as to deliver a smooth charge of fuel from
the pump jets by applying a pressure on the fuel that
remains constant through pump travel regardless of
speed or distance the throttle linkage is moved.
When the throttle valves are opened instantly to wide
open position, the spring loaded plunger will con-
tinue to supply fuel until the plunger reaches the
AS PLUNGER MOVES UP
FUEL FLOWS PAST CUP
INTO PUMP WELL .
PUMP RETURN
SPRING
DISCHARGE
CHECK BALL
A-1691
Figure 7-Accelerating Pump System