What is an Automatic Choke?

Lori Kilchermann

An automatic choke is a device that is mounted on the carburetor of a gasoline engine. When the ignition key is turned to the start position, this device is activated and the engine receives a much richer fuel mixture allowing the engine to start more easily. Once the engine has reached operating temperature, the choke is deactivated and the engine operates without the aid of a choke.

The carburetor mixes air and fuel and distributes this mixture to the engine.
The carburetor mixes air and fuel and distributes this mixture to the engine.

The carburetor mixes air and fuel and distributes this mixture to the engine. The engine then ignites this mixture and power is made. In cold weather, the engine requires more fuel than air to start properly. The choke mechanism blocks some of the incoming air from entering the carburetor. As the engine warms up, it is able to operate with more air and less fuel, so the choke is no longer needed.

Automatic chokes are typically mounted onto carburetors.
Automatic chokes are typically mounted onto carburetors.

Early gasoline engines used a manual choke mechanism. This manual choke was operated by pulling a knob in the operator's compartment. Once the engine was started, the operator pushed the knob in and the choke was eliminated. The automatic choke uses the vehicle's battery power to engage and disengage itself. This helps to eliminate the problem of leaving the choke on, which leads to poor fuel mileage.

The automatic choke is found only on carbureted gasoline engines. Fuel injected engines do not utilize a choke system to aid cold weather starts. Much like an automatic choke, a fuel infection system sprays more fuel into the combustion chamber at the time of start up. This allows the engine to run much richer and aid in cold weather starts.

Maintenance of the automatic choke is relatively simple. A bit of silicone spray on the choke plate shaft and spring at oil-change intervals will keep the automatic choke functioning properly. Dirt and debris can be removed from the choke unit with compressed air or a can of carburetor cleaner.

A common problem with automatic choke mechanisms is the failure to disengage when the engine warms up. This problem is easily rectified by a carburetor repair specialist. In most cases, a simple adjustment is all that is required to return the choke to proper working order. In some extreme cases, the choke mechanism must be replaced to ensure proper operation.

Have a competent mechanic service the choke at least once a year to prevent problems. Refer to the vehicle's operator's manual for proper operating instruction, as well as proper cold weather starting techniques.

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Discussion Comments


My car mixes gas with oil with black smoke.


My lawnmower does not have an automatic choke. It has a knob that I have to pull out before turning the key. If I don’t pull this knob out, the engine usually won’t start.

I have never tried operating my lawnmower in cold weather, but I’m sure the choke would help start it then, too. It needs the choke for starting if it has been awhile since I last cranked it. However, if I just turned the engine off to go inside a moment and then I crank it again, it will work without the choke.

Unlike an automatic choke, I have to push the manual choke knob in to turn it off. I like this, because I can be sure that it is off that way.

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