GMC 1976 ZEO 6083 Maintenance Manual page 345

through the calibrated idle channel restrictions, past
the lower idle air bleeds and off-idle discharge ports,
where it is further mixed with air. The air fuel mix-
ture moves down to the idle mixture needle discharge
holes where it enters the carburetor bores and blends
with air passing the slightly open throttle valves . The
combustible mixture then passes through the intake
manifold to the engine cylinders.
A fixed idle air by-pass system is used to supple-
ment the idle air passing by the slightly open throttle
valves . The purpose of the idle air by-pass is to
reduce the amount of air going through the carbu-
retor bores and still maintain sufficient air for the
correct idle speed. This reduces the amount of air
passing through the venturi system to prevent the
main fuel nozzles from feeding at idle . The venturi
system is very sensitive to air flow and where large
amounts of air is needed to maintain idle speeds, the
fixed idle air by-pass system is used .
The fuel tank will not vent to atmosphere, all fuel
vapors are collected in a vapor collection canister. A
timed purge port is provided in the carburetor throt
tle body above the throttle valves adjacent to the
off-idle discharge ports. The timed bleed purge holes
provide adequate purge to remove all vapors that will
be collected in the vapor canister . They will bleed
constantly during off-idle and part throttle operation
of the engine .
OFF-IDLE OPERATION
As the primary throttle valves are opened from
curb idle, additional fuel is needed to combine with
the extra air entering the engine . This is accom
plished by the slotted, off-idle discharge ports. The
primary throttle valves open gradually exposing the
off-idle ports to high engine vacuum below the throt-
tle valves . The additional fuel added from the off-idle
ports mixes with the increasing air flow past the
opening throttle valves to meet increased engine air
and fuel demands.
Further opening of the throttle valves causes low
pressure at the lower idle air bleeds . As a result, fuel
begins to discharge from the lower idle air bleed hole
and continues to do so from part throttle to wide
open throttle .
MAIN METERING SYSTEM (FIGURE 4)
The main metering system consists of main me-
tering jets, vacuum operated metering rods, main
fuel well, main well air bleeds, fuel discharge nozzles,
and triple venturi. The system operates as follows:
ENGINE FUEL SYSTEM
6M- 3
Figure 4-Main Metering System
During cruising speeds and light engine loads,
engine manifold vacuum is high . Manifold vacuum
holds the main metering rods down in the main me
tering jets against spring tension. Manifold vacuum
is supplied through a channel to the vacuum ope-
rated power piston connected to the primary main
metering rods . Fuel flow from the float bowl is me-
tered between the metering rods and main metering
jet orifice.
Primary throttle valves opened beyond off-idle
range allows more air to enter the manifold which
increases air velocity in the venturi. This causes a
drop in pressure in the large venturi and a much
greater drop in pressure in the small venturi. Low
pressure in the small boost venturi causes air fuel to
flow from the main discharge nozzle .
Fuel flows from the float bowl through the main
metering jets into the main fuel well . The fuel in the
main fuel well is mixed with air from the main well
air bleeds then passes through the main discharge
nozzles into the boost venturi.
The fuel mixture is combined with air in the
boost venturi into a combustible mixture and passes
through the throttle bores into the intake manifold .
CAUTION : An adjustable part throttle fea-
ture is incorporated in all carburetors. This
adjustment is made at the factory and no
attempt should be made to adjust it in the
field.
The adjustable part throttle features a power pis-
ton with a pin pressed into it, which protrudes
through the float bowl and gasket and contacts the
adjustable link in the throttle body . The primary
main metering rods have a double taper on the me-
tering tip and can be identified by the suffix "B"
stamped after the diameter on the rod. The purpose