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  5. 4100 Choke Circuit
The Autolite 4100, 4 barrel carburetor used an automatic hot stove type of choke. The choke housing is mounted on the carburetor near the top. Some may have been replaced by an electric choke, which is a good option due to the stove pipe connection is not easy to find.

4100 ChokeA- Thermostat coil. The tab on the spring fits into the slot at E. When the choke is closed (engine cold), the coil is coiled tighter. As the engine warms up the coil expands and assists in opening the choke valve. When the engine is at operating temperature, the choke valve should be open all the way.

B- Pull down piston. This is used to open the choke slightly when the choke valve is closed (engine cold) and the throttle is opened up. Without this, the choke valve would choke the engine causing it to die. The piston is moved by vacuum only so be sure it move freely. Clean it by spraying silicon spray lubricant into the hole. If frozen, then remove the clean out plug at F. Tap the piston to move it and clean out the hole thoroughly. Be gentle, the piston connects to the arm with a small pin, which will break (pin, or arm) easily.

C- Hot air tube connects here. Vacuum in the choke housing via the carburetor, pulls the hot air into the housing via the hole at D.
4100 Choke1964 & Later Carburetors

On some 1963 & earlier 4100 carburetors the center of the choke (C) has a spring loaded lever. Our thermostats do not fit these types.

Remove B to clean out the choke piston. Replace with our 85-48 aluminum plug. Insert the plug then hit the center with a hammer to expand it. This is enough to seal and you will not need to add any epoxy, but it won't hurt if you do.

A - This hot air inlet often will have worn threads. You can try this fitting to repair it. May not work on all depending on how worn it is.

Eliminate the hot air tube by using our electric choke conversion kit.
4100 Electric Choke
Autolite 4100 Choke Circuit


How The Fast Idle Works

Watch a video:



When starting your engine you would usually press the gas pedal down once so that the fast idle cam can move to fast idle. The idle screw also acts as the fast idle screw. When cold the screw ends up on the high side of the cam thus opening the throttle plates a bit which increases the RPM. Adjust the idle screw when cold to the fast idle specification in your motors manual.4100 Fast Idle


Once warmed up the choke valve is opened and the cam falls down to the low point. The idle screws lands on the low point allowing the throttle to close and you should be a idle RPM. If not adjust it to specs.

Replace your hot air choke with our electric choke kit:
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