long periods of engine idle. This is because the fuel
idle vent is activated by linkage from the throttle valve.
begins to evaporate in the float bowl and the vapors get
The idle vent system on later vehicles may be part of the
into the air horn through the balance tube. The solution
emission control system (para 7-11).
to this problem is to have an outside vent for the float
bowl that is opened whenever the enigine is idling. The
Section IV. SYSTEMS OF THE CARBURETOR
idle. As the throttle begins to open, the effectiveness of
4-15. General. The two operating systems of the
the idle circuit falls off gradually as the low-speed circuit
carburetor each contain two circuits that give them the
takes over. The transition between the two circuits is a
flexibility to operate throughout the entire engine speed
smooth one. Operation from engine idle through low-
range. Both of these systems obtain gasoline from the
speed range is as follows:
float bowl through the main jet (fig. 4-28). The main jet is
a precisely sized opening that helps govern the amount
(1) The throttle valve is almost closed at engine
of fuel used. The main jet usually is replaceable and is
idle. This creates a high vacuum in the area of the
available in a variety of sizes. Carburetors can be
carburetor under the throttle valve. This high vacuum
tailored to meet various needs by varying jet sizes. In
causes atmospheric pressure to push gasoline through
addition to the above, the carburetor must provide other
the idle port from the float bowl. The gasoline mixes with
systems to compensate for temperature change and for
the air that is drawn in around the throttle valve. The
quick changes in throttle position.
mixture then is drawn into the engine.
4-16. Idle and Low-Speed System.
(2) As the throttle valve is opened, the vacuum
under it begins to fall off, causing less gasoline to be
a. Purpose. The idle and low-speed system
drawn from the idle port. As more air flows through the
provides the proper air-fuel mixture when the engine is at
throat, the gasoline will begin flowing through the low
idle and during periods of small throttle opening. During
speed or off-idle discharge port, which is usually in the
these periods, there is not enough air flowing through the
shape of a rectangular slot or a series of two or three
throat to make the discharge nozzle work.
holes. During the low-speed system operation, there is
still not enough airflow through the throat for the
b. Operation (Fig. 4-29). The idle and the low-speed
discharge nozzle to work.
portions of the system are really separate circuits in
operation. The idle circuit sustains the engine at an
Figure 4-28. Main Jet.