Chevrolet Light Duty Truck 1973 Service Manual page 436

6 M - 1 2 ENGINE FUEL
TYPICAL FLOW CURVE SHOW ING
SYSTEMS OPERATION VS. M.P.H.
.
k V "
15=1
TRANSFER
1 IDLE&
1 MAIN
METERING
SYSTEMS
M IL E S PER
0 -2 5 2 5 -4 0
H O U R
Fig. 16-Typical Systems Operation
The main metering jet and the air bleeds are pre­
calibrated to maintain the desired air-fuel ratios
throughout the main metering range. Therefore, no
adjustments are necessary in the main metering system.
Power System
(Fig. 18)
As discussed previously, richer fuel mixtures are needed
for high speed operation since maximum engine power
requires the use of all available air for combustion. The
power system is used to supplement the main metering
system and provide the power enrichment according to
the amount of throttle opening and engine load.
The power system consists of: a power valve, power
piston and spring, and power restriction. The power
valve controls a fuel passage which bypasses the main
metering jet from the bowl to the main well. The power
piston is located above the valve and determines when
the valve will open.
The power piston is spring loaded and normally held in
the closed position by high manifold vacuum conditions.
Under conditions of power, manifold vacuum will be low
which means vacuum will decrease on top of the piston
allowing the spring to corce the power piston down,
opening the valve to increase the fuel flow to the main
nozzle. This has the same effect as enlarging the main
jet. A calibrated power restriction is used in the passage
between the power valve and the main well to control the
amount of enrichment.
While this system is constantly operative only at speeds
approaching wide open throttle and above, sudden
throttle openings at slow and mid-range speeds will cause
momentary opening of the power valve, due to resultant
manifold vacuum decreases. Power valve springs
generally are calibrated to allow opening to begin at
to 9" average and full opening when manifold vacuum
falls below 4" to " average. There is no adjustment
6
required for the power system.
7
MAIN METERING SYSTEM
SYSTEM
&
MAIN
MET'RNG.
4 0 -6 0
6 0 -M A X
"
8
Pump System
(Fig. 19)
When the throttle is opened rapidly, the air flow and
manifold vacuum change almost instantly. Because of
the great difference in weight between air and fuel, any
sudden change in throttle opening results in an
immediate increase in air intake but the fuel, having
greater weight tends to lag behind. The result of this is
momentary leanness. The accelerator pump provides the
extra fuel necessary to overcome this leanness and give
POWER
smooth operation during transient operations. This is
accomplished by discharging extra fuel into the venturi
air stream whenever the throttle valve is opened. The
pump system utilizes a pump plunger that is linked to
the throttle lever by mechanical linkage.
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 top
of the pump well, or through an inlet check valve in
earlier design carburetors. It flows past the pump cup
seal into the bottom of the pump well. The pump
plunger is the floating type of which the cup moves up
and down on the pump plunger head. Some models may
use the fixed cup, ball vapor vent design. 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. During a shut
down soak period 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. At the same time, the discharge check
valve seats to prevent air from leaking into the discharge
passage. When the throttle valves are opened, the
connecting 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 valve and passes on through the passage
to the pump jets where it sprays into the venturi area of
each bore.
The accelerator pump system is an accessory device
which has no function other than momentary operation
at the time of throttle position changes. The pump is
purely mechanical and not affected by air flow.
Choke System
(Fig. 20)
The necessary requirements for good fuel vaporization
are missing or are inadequate when starting a cold
engine. To overcome this condition, plus condensation of
the fuel on the cold manifold, it is necessary to meter
extremely rich mixtures from the carburetor (
air-fuel ratios) in order to provide sufficient combustible
mixtures to the cylinders for quick starting. This
enrichment is obtained by the addition of a choke valve
in the air horn above the main m etering system; starting
fuel to flow through these systems.
The choke system consists of a choke valve located in the
to
2:1 1:1
LIGHT DUTY TRUCK SERVICE MANUAL