Choke: This is incorporated in the main air passage and used to control the air quantity to supply rich mixture. While starting an engine, the choke shuts off the air supply to the carburetor so the resulting mixture will be rich in fuel and this opens automatically as the engine heats up.

The ideal carburettor passes a complete vaporized fuel air mixture in the proper proportions into the cylinder. A part of atomized fuel is vaporized in heated spots in the intake manifold.