Function of the Automatic Choke

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Note: The purpose of the choke is to reduce the air intake into the carburetor, thus enriching the fuel mixture so that the engine will start and run when cold. The automatic choke, located on the upper right side of the carburetor, consists of an electrically-heated bimetallic coil that warms up (hopefully) at the same rate that the engine warms up. As it warms, the bimetallic coil inside the choke opens the butterfly valve in the throat of the carburetor, allowing more air into the carburetor and thus increasing the percentage of air in the fuel-to-air mixture.

The choke can be adjusted in two ways. Procedure #1 below is the quick and easy way, simply utilizing the markings on the body of the choke. Procedure #2 is a bit longer but probably more accurate.


Procedure #1-

Be sure the hook on the bimetallic spring contacts the choke shaft lever.

Install the cover and retainer and lightly tighten the screws.

Turn the ceramic element to align the mark on it with the middle mark on the housing.

Procedure #2-

The engine must be cold to make this adjustment.

Remove the air cleaner.

Find the automatic choke. It’s the round thing on the upper right-hand of the carburetor. There is a wire coming to it from the positive side of the coil.

Note: The automatic choke is a round ceramic thing with the heating element wound inside of it. (The ceramic part may be covered with metal so that it looks just like the rest of the carburetor.) The choke is held in position by a triangular ring clamp and three screws so it can be loosened and rotated for adjustment.


Pull the throttle arm on the left side of the carburetor down to free the little step arm (cam) that the screw at the top of the throttle lever rests on.
Note: This stepped “cam” is connected to the butterfly valve inside the throat of the carburetor by a shaft that extends all the way into the automatic choke. With the engine cold, the butterfly valve should be closed. As the engine warms up, the coil inside the automatic choke uncoils, opens the butterfly valve, and moves the cam to reduce the idle speed.

Release the throttle arm so that the return spring snaps it back. The little screw at the top of the throttle arm (again, with the engine cold) should now rest on the top step of the stepped cam. This sets the hi-idle, which is needed together with the choke on a cold engine to provide sufficient idle speed to keep the engine running until it warms up fully.


Loosen the three screws on the right side that hold the choke in place.

Keep your eye on the butterfly valve in the carburetor throat.

Turn the choke element clockwise (viewed from the right) until the butterfly is standing straight up, then turn the choke counterclockwise (viewed from the right) until the choke butterfly fully closes (barely — not too tightly), then tighten the three screws that hold the choke in place.

Note: This is important; the automatic choke may be assembled wrong and not catching the hook on the coil spring at all.


Start the engine with the air cleaner off. As the engine warms up, make sure that the butterfly opens until it is standing straight up (full open) when the engine is fully warm. If it doesn’t, readjust the choke until you get it right.

Note: The engine is now warm, so you won’t be able to adjust the choke per the foregoing. Note the position of the notch on the side of the choke relative to the three little ridges on the body of the carburetor. If the butterfly is too far closed with the engine warm, turn the choke clockwise just a bit to straighten it up. The notch on the choke should never be too far outside of the three ridges on the body of the carburetor.

If you are not able to adjust the choke using these method, something may be sticking, or perhaps the coil spring inside the canister is broken, or perhaps the wire has fallen off of the contact on the canister so that it is not getting power from the battery properly.