GMC 1976 ZEO 6083 Maintenance Manual page 343

SECTION 6M
ENGINE FUEL SYSTEM
Contents of this section are listed below:
SUBJECT PAGE N0 .
Carburetor . . ... . . ... . ... . . . . . . . ... . .... . . .. . . . .. . . ... . . ... . . ... .... . ... . .... . ... . . . . . . . ... . . ... . . . ... . . . .. . . .... . . . . . . . .... . . .. . . . . . 6M-1
General Description . .... . . .. . . . .. . . ... . . ... . . .. . . . .. . . .. . . .. . . . .. . . .. . . . ... . . ... . . ... . . . .. . . . ... . . . .. . . . ... . . . .. . . . ... . . . 6M-1
Theory of Operation .. . .. .. . . .. . . . ... . .... . . .. . . . .. . . .. . . . . . . . .. . . .. . . . .. . . .... . . .. . . . ... . . . ... . . ... . . . ... . . ... . . . ... . . . 6M-2
Carburetor Diagnosis. . . ... . . ... . . . .. . . .. . . . ... . . ... . ... . ... . . ... . ... . . . .. . . .... . . .. . . . ... . . . .. . . . ... . . . ... . . ... . . .... . . . 6M-6
Carburetor Replacement ... . . ... . . .. . . . .. . . ... . . .. . . ... . .... . ... . . .. . . . .. . . .... . . . . . . . .... . . .. . . . ... . . . ... . . . .. . . . ... . 6M-14
Carburetor Adjustments . . ... . .... . ... . . . .. . . . . ... . . ... . ... . . ... . ... . . . .. . . .... . . . . . . . .... . . .. . . . ... . . . ... . .... . . . .. . . 6M-21
Accelerator Linkage . . ... . . .. . . . .. . . ... . . ... . . . . . . . .. . . ... . . .. . . ... . ... . . ... . . . .. . . ... . . ... . . . ... . . .. . . . . ... . . . .. . . .... . . . .. . . 6M-26
Fuel Pump ... . . ... . ... . . .. . . . .. . . .... . . .. . . . .. . . .. . . . ... . .... . ... . . .. . . ... . . .. . . . . . . . ... . . ... . . . .. . . . ... . . ... . . ... . . . .... . . .. . . . ... . 6M-26
Air Cleaner . . ... . . ... . . .. . . . .. . . ... . . ... . . ... . . .. . . . .. . . .... . .. . . ... . ... . . ... . . .. . . . .. . . .... . . . . . . . .... . . .. . . . ... . . . .. . . .... . . . ... . 6M-27
CARBURETOR
GENERAL DESCRIPTION
The Model 4MC (Quadrajet) is a 4 barrel carbu-
retor having two stages of operation.
The primary (fuel inlet) side has two small bores
each with a triple venturi which are equipped with
a discharge nozzle . Fuel is metered to the primary
bores by two tapered metering rods connected to a
power piston which is actuated by manifold vacuum .
The secondary side has two very large bores
which have greatly increased air capacity to meet
engine demands. The air valve opens as air velocity
increases and thereby controls the air/fuel mixture in
the secondary bores. This mixture combines with the
fuel mixture in the primary side . Using the air valve
principle, fuel is metered in direct proportion to the
air passing through the secondary bores .
The fuel reservoir is centrally located and uses a
single float pontoon.
A pleated paper fuel filter is mounted in the fuel
inlet casting of the float bowl and is easily removed
for replacement.
ENGINE FUEL SYSTEM 6M- 1
Carburetor identification information is located
as shown on Figure 1 . If replacing a float bowl as-
sembly, follow the directions received in the service
package to transfer the information .
The primary side of the carburetor has six sys-
tems of operation. They are float, idle, main meter-
ing, power, pump, and choke. The secondary side
has one metering system for controlling the air/fuel
mixture which combines with the primary side . The
primary and secondary side receive fuel from a com-
mon float bowl .
Figure 1-Carburetor Identification