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TM 9-8000
Figure 4-36. Duration Spring.
4-19. Choke System.
a. Purpose. When the engine is cold, the gasoline
closed so that the engine can be started. As the control
is pushed back in, the position of the choke valve is
tends to condense into large drops in the manifold rather
adjusted to provide the proper mixture. The following are
than vaporizing. By supplying a richer mixture (8:1 to
two features that are incorporated into manual choke
9:1), there will be enough vapor to assure complete
systems to reduce the possibility of engine flooding by
combustion. The carburetor is fitted with a choke system
automatically admitting air into the engine:
to provide this richer mixture.
The choke system
provides a very rich mixture to start the cold engine. It
(1)  A  spring-loaded  poppet  valve  that  is
then gradually makes the mixture less rich as the en-
automatically pulled open by the force of the engine
gine reaches operating temperature.
intake strokes.
b. Operation (Fig. 4-37). The choke system consists
(2) A choke valve that is pivoted off center on its
of a flat plate that restricts the throat above the venturi
shaft. This will create a pressure differential between the
but is located below the balance tube so that it has no
two sides of the choke valve when it is subjected to the
affect on the pressure In the float bowl. This plate is
engine intake, causing it to be pulled open against the
called a choke valve, and, like the throttle valve, is
force of spring-loaded linkage.
mounted on a shaft to tilt it opened or closed.
d.  Automatic Choke System (Fig. 4-39).  The
c. Manual Choke System (Fig. 4-38). The manually
automatic choke control system is centered around a
operated choke used to be the most popular way of
thermostatic coil spring. The spring exerts pressure to
controlling the choke valve. Due to emission regulations,
hold the choke valve closed. Heat is applied to the coil
the possible danger In use with catalytic converters (para
after the engine Is started. The heat causes the coil to
7-8), and technological advances in automatic choke
expand, allowing the choke to open.
systems, manual choke systems are little used today.
The choke valve is operated by a flexible cable that
e.  Providing Automatic Choke Heat.  The four
extends into the driver's compartment. As the control is
methods of providing controlled heat to the automatic
choke thermostatic spring are:


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