is a device in an internal combustion engine that atomizes and mixes fuel with air in the proper proportion for efficient combustion at all engine speeds, and controls the engine’s power output by throttling, or metering, the air-fuel mixture admitted to the cylinders. The automobile carburetor is a complex mechanism designed to compensate for many variables over a wide range of speeds and loads. Intake air is drawn through the venturi, a constricted throat in the air passage that causes a pressure reduction in the air stream, which draws fuel from the carburetor bowl through either the main jet or the idle jet. The fuel is atomized by the high-velocity air, and the resulting air-fuel mixture is piped through the intake manifold to the individual cylinders, where it is burned. A throttle plate between the venturi and the cylinders controls power and speed by controlling the volume of air-fuel mixture reaching the cylinders. In most carburetors, closing of this (venturi) throttle valve shuts down the main jet and activates the idle jet, which provides the fuel-rich mixture that idling requires. An accelerator pump in the carburetor provides momentary fuel enrichment when the accelerator pedal is depressed rapidly, to compensate for the sudden influx of air. During cold starting, a choke (or butterfly valve) restricts airflow to the carburetor, thus enriching the mixture for faster starting. The choke on most automotive engine carburetors is operated automatically by a thermostatic spring, which opens the choke as the engine warms up.

Also see fuel injection, supercharger.