The part that mixes air and fuel in a carbureted engine is the carburetor.
A carburetor meters fuel into the incoming air stream and creates a combustible air-fuel mixture before it enters the intake manifold. In a traditional design, air speeds up through a narrow passage called a venturi. The resulting pressure drop draws fuel from a bowl or metering circuit into the airflow.
Carburetors were central to gasoline-engine design for much of the automobile era. They could include separate circuits for idling, acceleration, high engine loads and cold starting. A choke temporarily restricts air during a cold start, enriching the mixture so the engine can run while fuel vaporizes less readily.
The intake manifold carries the prepared mixture toward the cylinders; it does not usually create the mixture itself. A throttle body controls airflow, while a fuel injector sprays fuel in an electronically controlled injection system. Since the 1990s, fuel injection has largely replaced carburetors in cars, though carburetors remain common in some small engines, motorcycles and piston aircraft.