during the warm-up period. An automatic choke system
has a choke plate (valve), a thermostatic spring, and
other parts depending upon choke design.
The choke plate is a butterfly (disc) valve near
the top of the carburetor air horn. When the choke plate
is closed, it blocks normal air flow through the
The thermostatic spring is a bimetal spring
(spring made of two dissimilar metals) which may be
used to open and close the choke. The two metals have a
different rate of expansion that make the spring coil
tighter when cold and uncoils when heated. This
coiling-uncoiling action is used to operate the choke.
The basic operation of the automatic choke system
is as follows:
With the engine cold, the thermostatic spring
holds the choke closed. When the engine is started, the
closed choke causes high vacuum in the carburetor air
horn. This pulls a large amount of fuel out of the main
As the engine and thermostatic spring warm, the
spring uncoils and opens the choke plate. This action
produces a leaner mixture. A warm engine will not run
properly if the choke were to remain closed.
Various methods are used to control the warming of
the choke thermostatic spring. The four methods of
providing controlled heat to the thermostatic spring are
electricity, engine coolant, well-type
heated, and exhaust manifold.
ELECTRICITY (fig. 4-33) uses an electric coil
to heat the thermostatic spring. The heating coil is
switched on with the ignition switch. Some systems use
a control unit that prevents power from reaching the
Figure 4-33.Electric choke.
Figure 4-34.Engine coolant heated choke.
electric coil until the engine compartment reaches a
ENGINE COOLANT (fig. 4-34) uses a passage
in the thermostat housing to circulate engine coolant for
heating the thermostatic spring.
WELL-TYPE HEATED (fig. 4-35) mounts the
thermostatic spring in the top of the exhaust manifold.
As the engine and manifold warms, the thermostatic
spring uncoils to open the choke.
The EXHAUST MANIFOLD (fig. 4-36) uses
heat from the exhaust manifold to heat the thermostatic
spring. The exhaust heat is brought to the choke
through the means of a heat tube. The heat tube passes
through the exhaust manifold, so as it takes in fresh air
via the choke stove, it picks up heat from the exhaust
without sending any actual exhaust fumes to the choke
When the choke system is operating during warm-
up, the engine must run at a faster idle speed to improve
drivability and prevent flooding. To accomplish this, fit
Figure 4-35.Well-type exhaust-heated choke.