How a Side Draft Carburetor Works

by John Albers
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What is a Side Draft Carburetor?

A side draft carburetor is a device integral to the functioning of almost all internal combustion engines constructed prior to the 1980s. Since then they've been replaced in the automobile industry by fuel injection systems, though they are still prominent in any small gas-powered engine such as on motorcycles and lawn care equipment. It is the task of a side draft carburetor to combine air with fuel in the correct ratio prior to its entry into the engine's combustion cylinders. Because this ratio changes depending on how rapidly the engine is functioning, the side draft carburetor must be self-adjusting as well.


A side draft carburetor, as the name would suggest, is located on the side of an engine. This is opposed to the more common down draft carburetor which is found atop the engine. In either case, the carburetor is made up of a filtered air intake running to the cylinders. Along this intake line is a narrow, pin-sized hole. This leads back to a narrow metal tube called a venturi. There is a solid metal housing from the venturi to the vehicle's fuel supply line. Closing off the housing from the venturi is a valve called a throttle plate. This throttle plate is spring loaded and connects directly to the throttle assembly of the engine.

How Does a Side Draft Carburetor Work?

Liquid fuel sits behind the throttle plate. As the engine starts, the throttle plate slowly opens. It opens wider and wider as the engine's throttle is depressed. This lets more fuel through, past the venturi into the jet line. Because the jet is so narrow, the air from the air intake passing by its far end creates a vacuum, pulling the fuel in atomized particles down with the flow of air into the engine's combustion cylinders. Because the throttle will dictate the engine's fuel/air ratio needs, the throttle plate will always be open to the required degree to ensure the optimum ratio is provided.

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