Classic cars that predate electronically managed fuel injection often use a manual choke to vary the mixture of fuel and air. A manual choke is a mechanical device that links a retractable knob in the driver's compartment, via a cable, to a metal plate inside the carburettor. The metal plate moves to increase or decrease the amount of intake air in order to provide the optimum fuel mixture for the engine. Effective use of a manual choke requires an understanding of the mechanism, and the effects of temperature changes on fuel.
Pull the manual choke knob to decrease the air intake and provide a richer fuel-to-air ratio before you start the ignition. Adjust the amount of choke you pull out depending on the engine temperature. More fuel vapour is present in a hot engine, which requires less choke.
Pull out the choke knob more for a cold engine start, or on a cold day. More fuel droplets and less fuel vapour are present in a carburettor on a cold day, so you require more choke.
Switch on the ignition and start the engine. Adjust the fuel-to-air ratio of the fuel mixture delivered to the car engine by pushing the manual choke slowly as the engine heats up to operating temperature.
Listen to the engine to determine whether you need to fine tune the manual choke position. Adjust the choke position if you hear the engine make a stuttering or sputtering noise.
Pull out the choke slightly to check if the engine requires more fuel. Push the choke in slightly to check if the engine requires more air. Listen to the engine after each adjustment to hear the choke's effect on the engine to determine the correct choke position for a smooth noise.
Push the manual choke in gradually as the car engine warms up. Push the manual choke in fully once the car engine reaches a normal operating temperature, usually just a few minutes.
The car engine may stall if you apply too little or too much manual choke.