The carburetor supplies the engine with the correct air-fuel mixture for all conditions of operation.
Carburetors come in different designs.
This is a basic downdraft carburetor, with a fixed-size venturi. Fuel is pumped to the float bowl where a float and a needle valve control its level. The float bowl is open to the atmosphere. The barrel of the carburetor has an air horn and a venturi. A throttle valve below the venturi is linked to the accelerator pedal. Its action allows airflow into the carburetor barrel.
The shape of the venturi constricts air-flow and lowers pressure there. The pressure difference between the bowl and the venturi causes fuel to spray out of the fuel discharge nozzle. It mixes with the air, and the mixture passes the throttle valve and enters the intake manifold, and the engine.
Depressing the accelerator increases air speed through the carburetor. This lowers air pressure at the nozzle. Atmospheric pressure on fuel in the float bowl stays constant, so more fuel is forced into the venturi to mix with the increased air. This keeps the air-fuel ratio roughly constant for a range of throttle openings.
The throttle valve also controls flow of mixture into the engine. Opening it lets more mixture be delivered, which increases engine power and speed. Closing it has the opposite effect.
