The carburetors is a device used to mix, or meter, fuel with air in proportions that satisfy the energy demands of the engine in all phases of operation. A carburetor is a very complex mechanism. Some of the larger two- and four-barrel carburetors can have over 200 parts. These parts make up the metering systems and subsystems that are necessary for matching air and fuel delivery with engine performance demands. Each of these systems must be functional and properly adjusted if the engine is to operate efficiently.

Although fuel injection systems have replaced carburetion in all passenger cars and light trucks, there are many carbureted engines still on the road. Technicians must understand the principles of carburetion and how carburetors are constructed and operate before they can successfully diagnose and tune carbureted engines. Carburetors have a venturi and work under the principle of pressure differential. The amount of air and fuel delivered to the engine depends on the difference between the low pressure (vacuum) in the engine and the pressure of the outside air (atmospheric pressure).