The typical small engine carburetor also has a choke, which may be a round plate hinged in the air horn, or a disc that can be pivoted to cover the air horn at the top. In either case, closing the choke plate restricts the air flow through the air horn, so that the air-fuel mixture is exceptionally rich with fuel. This is necessary for cold starting on small gas engines, as it

is on large passenger car powerplants. The reason for the choke involves the components of gasoline and the metal temperatures of the fuel system and engine. For best combustion the fuel must be capable of becoming a well-mixed vapor very quickly. In a warm engine, the heat emitted by the engine can vaporize the fuel very easily. In a cold engine, however, vaporization is a problem. Gasoline actually is a blend of many types of fuel, some of which vaporize easily, others of which do not. When the mixture is made very rich with fuel, there will be enough of these easily-vaporized

components to form a mixture that will burn and produce adequate power when the engine is cold.

   The carburetor also may have three fuel jets projecting into the air horn, one for low-speed operation, another for medium-speed (rare) and a third  for high-speed.

   To insure that the air entering the carburetor and engine is clean (since abrasives in the air cause engine and carburetor wear), an air filter is mounted on top of the carburetor air horn. This filter must be clean. If clogged with dirt particles, it restricts the flow of air (and fuel) into the carburetor, thereby reducing the maximum possible performance.

  Fuel system operation is somewhat tiresome to many. This concludes the theoretical section in this chapter. Those who wish some more detail, with some specific examples, will find it in Chapter 7.

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