Pontiac FIREBIRD 1972 Service Manual page 668

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6B-14 1972 PONTIAC SERVICE MANUAL
Six basic systems of operation are used: float, idle, main lowing text describes the purpose and operation of each
metering, power enrichment, pump and choke. The fol-
system.
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OPERATING SYSTEMS
FLOAT SYSTEM
The float system (Fig. 6B-14) controls the amount and
level of the fuel in the carburetor float bowl. Higher than
specified fuel levels can cause flooding, hard, hot starting,
rich fuel mixtures causing poor economy, nozzle drip at
idle and stalling. Therefore, it is important that the float
be set to recommended specifications.
The float system on the Monojet carburetor is located
adjacent to the main venturi. It is designed so that angular
maneuvers such as steep hills and sharp turns will not
affect proper operation by keeping an adequate supply of
fuel in the bowl at all times. The float system consists of
the following: a fuel inlet filter and
relief spring,
a solid single pontoon float made of special lightweight
plastic, a conventional needle and seat and a float hinge
pin. The float hinge pin fits in dual slots cast in the float
bowl and is held in place by compression of the air horn
gasket against the upper loop of the hinge pin.
The float operates as follows: fuel from the engine fuel
pump is forced through the paper fuel inlet filter, located
behind the fuel inlet nut, passes from the filter chamber
up through the float needle seat and spills into the float
bowl; as the float bowl fills with fuel, it lifts the float
pontoon upward until the correct fuel level is reached in
the float bowl. At this point, the float arm forces the float
needle against the float needle seat, shutting off fuel flow.
INTERNAL VENT
PRESSURE
RELIEF VALVE
F ~ T E R
RELIEF
SPRING
Fig. 68- 1 4 Float System
As fuel is used from the float bowl, the float drops down-
ward, allowing the float needle to move off its seat and
more fuel to enter the float bowl. This cycle continues
throughout engine operation, constantly maintaining a
positive fuel level in the float bowl.
The fuel inlet filter has a pressure relief spring located at
the rear of the filter. It seats between the rear of the filter
and the inlet casting. Should the filter become clogged
from improper servicing or excess dirt in the system, the
relief spring lets the filter move off its seat. This prevents
complete stoppage of fuel flow to the carburetor until the
filter can be replaced.
The carburetor float chamber is internally vented through
a hole located in the air horn above the float chamber. The
purpose of the internal vent is to balance air pressure on
the fuel in the float bowl with carburetor inlet air. With
this feature, a balanced air/fuel mixture ratio can be main-
tained during part throttle and power operation because
the air pressure acting on the fuel in the float bowl will be
balanced with the air flowing through the carburetor bore.
The carburetor external idle vent valve has been removed
and is replaced by a pressure relief valve. Should excessive
vapor pressure build up in the float bowl during periods
of hot engine idle or hot soak, a small valve at the top of
the air horn will be pushed off its seat, allowing the pres-
sure to be relieved, thereby preventing fuel from being
forced from the float bowl into the engine.
IDLE SYSTEM
The purpose of the idle system (Fig. 6B-15) is to control
fuel mixtures to the engine during idle and low speed
operation. The idle system is needed during this period
because air requirements of the engine are not great
enough to obtain efficient metering from the main dis-
charge nozzle and venturi system.
The idle system consists of a removable idle tube, idle
passages, idle channel restriction, idle air bleeds, slotted
off-idle port, idle mixture adjusting needle and the idle
mixture discharge hole.
During curb idle, the throttle valve is held slightly open
by the idle stop screw. The small amount of air, which
passes between the throttle valve and bore, is regulated by
this screw to provide the correct engine idle speed. Since
the engine requires very little air and fuel for idle and low
speed operation, fuel is mixed by direct application of
engine manifold vacuum to the idle discharge hole just