When an engine is cold, it requires a richer Air/Fuel Mixture to get started. A choke is used to create that condition.
The choke is a plate or blade that covers the primary barrels. It restricts airflow through the carburetor. This means that there is more fuel and less air entering the intake manifold.
As the engine warms up, it is able to run on a leaner mixture. The choke plate must be opened gradually to allow more air into the engine.
Carburetors are available with or without a choke. There are also several types of chokes to choose from.
How is it operated?
Manually - A manual choke is controlled by a lever on the side of the carb. A lever or knob inside the vehicle is then attached by a cable. This requires a person inside the car to slowly open the choke by hand.
Automatically - An automatic choke uses a metal spring to open and close the choke plate. The spring is wound in a housing and attached to the choke linkage on one end. As the engine warms up, it warms the metal spring. As the spring warms, it expands, rotates, and opens the choke plate.
Automatic chokes can be 1 of 3 types:
- Electric Choke - An electric choke uses electricity to warm the spring and gradually open the choke.
- Divorced Choke - In a divorced choke set-up, the metal spring is located in the intake manifold. The spring connects to the carburetor with a small rod. The spring is warmed by exhaust gases flowing through the crossover passage.
- Hot-Air Choke - In a hot air choke set-up, the metal spring is located in its own housing. A tube connects to the housing and supplies air heated by the exhaust.
How does it affect performance?
If you live in a warm climate you may not need a choke. Also, most race vehicles will use a carb without a choke.
If you do require a choke, you can choose the one that best suits your needs. If you want more control, you can choose a manual choke. An automatic choke can be tricky to install and adjust. But, it is more convenient than a manual choke.
Divorced and Hot Air chokes are often used when replacing an OEM style carb. If the engine is already set-up to operate one of these chokes, it is easy to keep it that way.
Electric chokes are popular and work very well. They are also easy to install and maintain. Electric Choke Conversion Kits are available for many applications.