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TM 9-8000
Figure 4-29. Idle and Low-Speed Systems.
c. Idle Mixture Screw (Fig. 4-29). A needle- shaped
discharge nozzle, where It mixes with the air In the
venturi.  Opening the throttle valve and accelerating
screw Is used in the carburetor to regulate the Idle port
engine speed Increases the airflow in the venturi, which
opening.  The air-fuel ratio of the Idle system can be
causes a proportional increase in the amount of gasoline
adjusted by turning the screw In or out.
from the discharge nozzle. The high-speed enrichment
d. Air Bleeds (Fig. 4-29). Air bleeds also are used in
system Increases the fuel flow to the discharge nozzle by
either Increasing the main jet opening or providing a
the idle and low-speed circuits to help atomize the fuel.
second supply of fuel from the float bowl.  There are
three basic high-speed enrichment systems that are
e. Passage to Float Bowl (Fig. 4-29). The passage
covered In para- graphs 4-17c, d, and e.
that supplies the Idle and low-speed circuits must at
some point be higher than the level of the gasoline In the
c. Power Jet (Flg. 4-31). The power Jet system
float bowl. If this passage went straight to the Idle and
Includes a Jet that Is opened by a vacuum- operated
low-speed ports, the float bowl would be able to drain
piston. The Jet provides an extra supply of fuel to the
through them.
discharge nozzle from the float bowl. When the throttle
valve Is not opened wide, there will be high manifold
High-Speed and High-Speed Enrichment
vacuum because the carburetor throat is restricted. This
high manifold vacuum Is used to hold the vacuum piston
against its spring. When the piston is up, the spring in
a.  Purpose.  The high-speed circuit supplies the
the power jet will hold it closed.  The throttle valve is
fuel-air mixture to the engine during medium to full
opened when extra power is demanded, causing a drop
throttle valve opening. The high-speed circuit gradually
in manifold vacuum.  As manifold vacuum drops, the
will take over from the low-speed circuit as the throttle Is
spring on the vacuum piston pushes the piston down,
depressed.  The carburetor Is designed to provide
which in turn pushes the power valve open. The power
approximately a 15:1 to 17:1 air-fuel ratio under normal
jet sometimes is referred to as the economizer and the
steady speed conditions.  The high-speed enrichment
vacuum piston is referred to as the step-up or power
circuit will enrich the mixture to approximately 11:1 to
12:1 if a heavy demand is placed on the engine.
d. Vacuum-Operated Metering Rod (Fig. 4-32). The
b.  Operation (Fig. 4-30).  The high-speed circuit
vacuum-operated metering rod uses a rod with a
takes Its gasoline from the float bowl through the main
diameter that gets progressively
jet. The gasoline is fed through a passageway to the


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