Hi all, there have been a lot of questions going around about the automatic choke - how it works, what it's supposed to do, why yours isn't working, etc. Instead of trying to tackle each one in each thread, I thought I'd start a new thread to cover everything you need to know about that mysterious black cap on the passenger side of your carburetor. Hopefully this will take care of all the doubts, frustrations, mysteries and questions that have been floating around the forum.
What that choke does.
The carburetor's job is to control the air\fuel mixture delivered to the engine. When the engine is cold, the engine needs to run rich, which means the amount of atomized fuel in the mixture needs to be higher than you'd normally have when the engine is warmed up. The carburetor accomplishes this by using a choke plate at the very top of the throat of the carburetor. When the engine is warmed up and running under normal conditions, the choke plate stands completely vertical and incoming air moves right past. However, when the engine needs to run rich, the choke plate can rotate to close off the throat of the carburetor and restrict the volume of incoming air. However, because the vacuum signal coming from the intake is still just as strong, this increases the negative pressure in the throttle body of the carburetor, which pulls more fuel from the bowl. That's basically it.
The choke plate is opened and closed through a mechanical linkage which is controlled by the black cap on the passenger side of the carburetor. This is called the choke cap, choke spring, or choke thermostat. Inside the choke cap is a spring. It is bimetallic meaning it is made of two different metals. Two long pieces of metal are fused on top of one another and wound into a spring. The two metals have different coefficients of thermal expansion which means as they get hot, one gets bigger than the other. Because they are attached on top of each other, the spring literally unwinds when it gets hot. It doesn't unwind much, but it's enough to make the end of the spring (often called the tang) to move several degrees. The tang is situated inside a slotted arm which, through the choke linkage, is connected to the choke plate. As the choke spring heats up, the spring unwinds, which in the end opens the choke plate. By the time the choke is as hot as it's going to get, the choke plate is completely vertical and wide open.
That's a lot of words to describe a simple concept, but that's how it works. So you know that the important thing here is heat - you have to get the choke hot to get the choke plate to open up. Let's move on to how it gets hot, because that's where most of the questions are coming from.
If you have a factory Motorcraft 2-barrel carburetor with an automatic choke, then you at least have a hot-air stove running things. This setup is often referred to as a hot-air choke. This literally pulls warm air from an exhaust crossover into the space inside the choke spring to warm it up. I'll start with a general picture, and move on from there.
This picture shows the passenger side of the carburetor. Each component of the system is explained below.