GMC 1976 ZEO 6083 Maintenance Manual page 344

6M- 2
ENGINE FUEL SYSTEM
THEORY OF OPERATION
FLOAT SYSTEM (FIGURE 2)
The float system consists of a float chamber, plas-
tic float pontoon assembly, float hinge pin and re-
tainer combination, a float valve and needle assembly
and a needle valve pull clip . The float system oper-
ates as follows:
Fuel from the engine fuel pump enters the carbu-
retor fuel inlet passage. It passes through the filter
element and on into the float inlet valve chamber.
The open needle valve allows fuel to enter the float
bowl .
As incoming fuel fills the float bowl to the pre-
scribed fuel level, the float pontoon rises and forces
the fuel inlet valve closed, shutting off all fuel flow .
As fuel is used from the float bowl, the float drops
and allows more incoming fuel to enter the float bowl
until the correct fuel level is reached. This cycle
continues constantly maintaining a positive fuel level
in the float bowl .
A needle valve pull clip is used to assist in lifting
the needle valve off its seat whenever fuel pump pres-
sure or the fuel level in the float bowl is low.
A plastic filler block is located in the top of the
float chamber in the area just above the float valve .
This block prevents fuel slosh on severe brake ap
plications maintaining a more constant fuel level to
prevent stalling.
The carburetor float bowl is internally vented .
Internal vent tubes are located in the primary side of
the carburetor air horn just above the float bowl . The
purpose of the internal vents is to equalize the air
Figure 2-Float System
pressure on the fuel in the float bowl with the air
pressure within the air cleaner. Therefore, a balanced
air/fuel mixture ratio can be maintained during part
throttle and power operation because the same pres-
sure acting upon the fuel in the float bowl will be
balanced with the air flow through the carburetor
bores. The internal vent tubes allow the escape of fuel
vapors in the float bowl during hot engine operation .
This prevents fuel vaporization from causing rich
mixtures due to excessive pressure in the float bowl .
IDLE SYSTEM (FIGURE 3)
The idle system is only used in the two primary
bores of the carburetor . Each bore has a separate idle
system . They consist of: Idle tubes, idle passages, idle
air bleeds, idle channel restrictions, idle mixture nee-
dles, and idle discharge holes. Idle mixture screw
limiter caps are installed on all carburetors. The
screws are preset at the factory and
SHOULD NOT BE
REMOVED .
During curb idle, the primary throttle valves are
held slightly open by the throttle stop screw to give
the engine the desired idle speed. Since the engine
requires very little air for idle and low speeds, the idle
discharge holes below the throttle valves are exposed
directly to engine manifold vacuum . With the idle
discharge holes in a very low pressure area and the
fuel in the float bowl vented to atmosphere (high
pressure), the idle system operates as follows:
Engine manifold vacuum at the idle discharge
ports causes fuel to flow from the float bowl through
the primary metering jets into the main fuel wells.
The fuel is picked up and metered at the lower tip of
the idle tubes. It passes up through the idle tubes,
then through a cross channel in the air horn casting
to the idle down channels where it is mixed with air
at a side idle bleed locatedjust above the idle channel
restriction .
The mixture continues downward
Figure 3-Idle System