Lesson 1/Learning Event 2
One method is a choke valve that includes a spring-loaded poppet valve. The poppet is normally held in the
closed position by a weak spring. As soon as the engine is cranked, a very high vacuum is created under the
choke valve. This allows the higher outside air pressure to open the poppet valve and permit air to enter the
In the manual method, the choke valve is operated by a coiled spring on the choke shaft. With the choke
closed and the engine cranking, air pressure will overcome the spring tension and open the choke valve
slightly to supply some air. As the engine warms up, the driver gradually opens the choke to supply a leaner
An automatic choke controls the air-fuel mixture for quick starting at any temperature.
In addition, it
provides the proper amount of choking during the entire engine warm-up period.
The automatic choke has a thermostatic spring and a vacuum piston to control the position of the choke
valve. The spring tries to close the choke valve when the engine is cold, while the vacuum piston tries to
open the valve. The choke valve is mounted off-center on its shaft, and any increase in air speed aids in
opening the choke valve more.
The operation of the automatic choke depends on three factors: temperature, intake manifold vacuum, and
speed of the air passing through the carburetor throat. Temperature change affects the thermostatic spring;
as the spring is heated, it loses its tension and allows the choke to open.
When the engine is cold, the spring holds the choke valve closed. When the engine is started, vacuum from
below the throttle valve is directed to the bottom of the vacuum piston. High vacuum under the piston
causes the piston to work against spring pressure and partially open the choke. A small amount of air,
heated by the engine exhaust, passes through the choke housing to heat the spring, allowing the choke to
open as the engine warms up.
Different load conditions will also cause the position of the choke valve to change. A decrease in manifold
vacuum will allow the spring to close the choke against pressure of the vacuum piston. An increase in air
speed through the carburetor throat will force the choke valve open against tension of the spring.