Chevrolet 70 1969 Series, Shop Manual

Page: 334 / 913

ENGINE FUEL SYSTEM 6 M -7
IDENTIFICATION
Carburetor has an identification part number
tag on the air horn cover, adjacent to the fuel
inlet. Before attempting to repair or overhaul
carburetor, refer to the identification number and
secure the correct repair kit.
OPERATION
The following paragraphs apply to all carbur­
etors except as otherwise indicated.
F L O A T S Y S T E M
The function of the float system is to maintain
a constant level of fuel in the float chamber at all
times and under all conditions of operation. Fuel
enters the carburetor at the fuel inlet, flowing
through the float needle valve and seat into the
float chamber as shown in Figure 8. When the
fuel reaches a given level, the float shuts off the
fuel supply at the needle valve. The float chamber
is vented internally by a vent tube which connects
the float chamber with the air horn.
ID LIN G S Y S T E M (Views A and B, Fig. 8)
With the throttle valves closed and with
engine running at slow idle speed, fuel from the
float chamber is metered into the idle tubes by an
orifice at the base of each idle tube as shown in
views A and B, Figure 8. The air taken in through
the idle air bleed holes, mixes with the fuel at the
top of the idle tubes. This mixture of air and fuel
then flows down the channels where it is mixed
with additional air entering through the secondary
idle air bleeds. This mixture is then discharged
at the lower idle discharge holes. The quantity of
fuel discharged is controlled by adjustable needle
valves. As the throttle valves are opened slightly,
the air-fuel mixture is also discharged from the
upper idle discharge holes to supply the additional
fuel required for increased engine speed as shown
in View B, Figure 8.
M A IN M E T E R IN G S Y S T E M (View C, Fig. 8)
The main metering system controls the flow
of fuel during the intermediate or part throttle
range of operation. With the throttle valves in a
partially opened position, fuel flows from the float
chamber through the main metering jets and into
the main discharge jets where it is mixed with air
taken in through the high speed air bleeders shown.
This mixture of air and fuel is then discharged
into the air stream through the auxiliary venturi
tubes. The main body and main discharge jets are
so designed that should vapor bubbles form in the
fuel in the main discharge system, due to high tem­
peratures, the vapor bubbles will collect in the
outside channel surrounding the main discharge
jets, rise, and vaporize in the domes of the high
speed bleeders thus presenting percolation reaction.
P O W E R S Y S T E M (View D, Fig. 8)
The power system is incorporated into the
carburetor to provide a richer mixture for maxi­
m um power and high speed operation. The extra
fuel for power is supplied by a vacuum-controlled
power piston which automatically operates the
power by-pass jet in accordance with throttle open­
ing. Intake manifold vacuum is maintained above
the vacuum piston through a vacuum channel which
leads to the mounting flange of the carburetor as
shown. During part throttle operation, the vacuum
above the vacuum piston is sufficient to overrule
the compression spring and hold the piston in the
up position. When the throttle valves are opened to
a point where the manifold vacuum drops to ap­
proximately four to five inches Hg., the compres­
sion spring then moves the piston downward to
open the power by-pass jet and meter additional
fuel into the main metering system.
A C C E L E R A T I N G S Y S T E M
(Views E and F, Fig. 8)
To insure smooth, uninterrupted power for
acceleration, additional fuel must be metered into
the engine. This is accomplished through the use
of an accelerating pump, which is operated by the
throttle linkage. As the throttle valves are opened,
the accelerating pump piston moves down to close
the inlet ball check valve and force a metered
quantity of additional fuel through the outlet ball
check valve and pump discharge nozzle into the
air stream, as shown in View E, Figure 8.
When the throttle is again closed, the piston
is raised against the compression of the pump
spring. When the throttle is opened the pump lever
moves down and permits the compression spring
above the piston to move the piston down. The
calibrated spring then delivers the metered quan­
tity of fuel over a given period of time.
With the release of the accelerator pedal and
the return of the accelerating pump piston to the
release position, the outlet ball check valve closes
while the inlet ball check valve reopens, thus per­
mitting fuel from the float chamber to enter and
refill the accelerating pump cylinder as shown in
View F, Figure 8. The inlet valve closes as the
accelerating pump piston reaches the release
position.
C H O K E S Y S T E M
The choke plate is the means used to provide
an enriched flow of fuel to aid in starting and
warming-up a cold engine. Closing choke plate,
CHEVROLET SERIES 70 -80 H E A V Y DUTY TRUCK SH O P M AN U AL