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 carburetor.

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 discharge tube.

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 as follows: 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 desired temperature.

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 mechanism.

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.