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Do-It-Yourself Projects
All Choked Up
By Iain Ayre/
Printable version
Degree of DifficultyDifficult
Estimated Time180 minutes
180 minutes
Switching from an Automatic to a Manual Choke -
Every problem is an opportunity, they say, but the best way of dealing with the problem of a faulty automatic choke is to take the opportunity to throw it in the bin. If you run an older car, you may well be stuck with an auto-choke system. However, if your car is old enough to be a classic rather than merely old, it probably has a traditional manually operated choke with a cable pull. And there are some good reasons to consider reverting to that type of system, as we shall see.
Switching from an Automatic to a Manual Choke
What Is It?
The principle of a choke is simple enough. When an engine is cold, it requires slightly more fuel in the air-to-fuel mixture during the first few minutes of operation. Typically, a choke mechanism will operate a butterfly in the inlet tract of the carburetor to restrict the amount of incoming air, which richens the mixture by reducing the percentage of airflow. There may also be a mechanism to raise the idle speed slightly. This system is still used in the latest cars, but nowadays it works by pumping a little more fuel through the injectors using a separate choke thermostat, and by raising the idle speed electronically.
The automatic choke works by using a bimetallic strip. Different metals expand at different rates when heated, so if two strips of metal are attached to each other, the resulting double strip will bend when it is heated up. The bimetallic strip inside an automatic choke is shaped into a coil, and is set to keep the choke on when it's cold and to release the choke as the engine and the strip warm up.
Why Switch?
There are two problems with this design. Firstly, if a bimetallic strip is bent twice a day for 30 years, it will eventually break. Secondly, you only need the choke for the first quarter-mile or so, and automatic chokes usually keep pouring extra fuel in way past the point where it's needed, wasting a lot of gas. You may be able to get a replacement for a failed automatic choke, but you also have the option to get rid of it.
Essentially, all you're doing is replacing the curling movement of the bimetallic strip with a cable pull in the car's cabin. Pull the choke to start the engine, then ease it back in as soon as the engine begins to run smoothly without it. This will save money and will operate reliably as long as your fingers remain reliable.
The bimetallic strip is coiled up inside the choke casing on the side of the carburetor. The casing is held in by rivets that will usually have to be carefully drilled out.
The Bimetallic Strip is Coiled Up Inside the Choke Casing on the Side of the Carburetor
Once the rivets are removed, the remaining hole in the carb will be filled with the replacement manual choke mechanism.
The Hole in the Carb will be Filled with the Replacement Manual Choke Mechanism
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