The main purpose of the carburetor is to control the mixture of fuel vapor and air that goes into the engine. The carburetor changes this ratio depending on the car's situation. It puts in a small amount of a very rich fuel mixture when the engine is cold and running at idle. With the throttle plate closed and air limited because of the closed choke plate, engine suction is amplified. More fuel is provided when the gas pedal is depressed for acceleration.