Marvel Schebler Choke System

The choke system is used during old starting and the warm-up period. Under these cold conditions it is necessary to supply an additional rich mixture of fuel and air, as only the “light ends” or more volatile portions of the fule will vaporize with the manifold and air temperatures at these cold conditions. Consequently it is necessary that a large quantity of fuel be available so that there will be enough “light ends,” to combine with the air to form a combustible mixture for starting the engine.

The function of the choke valve (25) is to restrict the amount of air that can enter the carburetor and to increase the suction on the nozzle (14) so that additional fuel will be drawn into the manifold. As soon as the engine fires and runs the rich mixture must be rapidly reduced to prevent stalling. This change in mixture is accomplished by the operator positioning the choke valve to provide the proper mixture. However, a few degrees movement of the choke valve will make a big change in the mixture strength and to help reduce the sensitivity of the choke valve position, use is made of a spring loaded relief valve (26) in many applications. This valve opens automatically with engine speed and load and eliminates a great deal of manipulation of the choke on the part of the operator.

When the engine has obtained normal operating temperature the choke valve must be fully opened to assure maximum power and economy. In addition, extended use of the choke results in more gasoline being supplied to the engine than can be burned. A large percentage of the unburned gasoline is lost through the exhaust system. The remainder of the raw gasoline is forced between the pistons and cylinder walls, washing away the protective oil film and increasing engine wear, and enters the crankcase where it dilutes the engine oil.

Any adjustments that are necessary on the carburetor should never be attempted until the engine has obtained normal operating temperature and the choke valve has been placed in a wide open position.

Rich Condition
Check the relief valve. It may be stuck closed. The spring may be missing or broken. The spring is not available, so when missing a more manual control of the choke will be necessary.
Fuel leaking out of the throttle shaft. When cold too much choking will cause this to happen. When the engine is warm and the choke is open there should be no fuel coming out of the throttle valve. If so, then you have a flooding condition.

Carburetor Parts
Updated on 05/30/2022

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