We have 4 different types of chokes, Intregal, Divorced, Electric & Electric Conversion.
The integral choke is mounted on the carburetor, either near the top, or near the bottom. The illustration shows the choke mounted on the top of the carburetor. The thermostat is heated by a heat tube that runs down to the exhaust manifold. The heat tubes heats up the thermostat and moves the choke valve to the open position. A vacuum passage in the carburetor feeds up to the choke housing and helps pull in the heat from the heat tube. Some of these chokes will also have a hot water jacket running through, or on the thermostat to facilitate heating.
With this type of choke, the choke is not opened by the thermostat. For the most part when warm, the choke opens from the weight of the linkage hooked to it. Along with that, the vacuum from inside the carburetor pulls it open.
The divorced choke type has the thermostat mounted in the intake manifold instead of on the carburetor. In this illustration, the arrow points to the thermostat which is covered by a metal shield. A rod connects the thermostat to the choke lever, which controls the choke valve to be opened, or closed. As the intake manifold heats up, the thermostat mounted, expands, opening the choke valve.
This is a typical electric choke mounted on the carburetor, which is integral. In this case there is one wire on the choke thermostat. This is the 12v source, which all electric chokes use. The thermostat is grounded via the carburetor itself. Some electric chokes will have a 2nd wire (ground wire), which is connected back to the carburetor. When the key is on the thermostat is heated up, opening the choke valve.
Electric Choke Conversion
This is a typical electric choke conversion kit. This is used to convert an integral, or a divorced choke to an electric choke. These choke kits come with a temperature gauge that bolts on to the intake manifold, which provides more accurate choke control than with an electric choke without the gauge. The electric choke conversion kit is not used for a manual choke conversion. The choke housing must be in place in order for it to work.
Electric choke thermostats need 12v or power. Anything less will work but the choke will open slower than it should. 6v does not work. Use any 12v source that is hot only when the key is turned on.
Putting a fuse in line with the thermostat is OK, but the thermostat will work without one.
No resister should be used in line with the thermostat.
How a Hot Air Choke Works
When the engine is cold a richer fuel mixture is needed. As it warms up the mixture must be leaned out. This is where the automatic choke circuit comes into play.
Choke Thermostatic Coil
The choke shaft extends through the carburetor into a round housing. Inside the housing there is a thermostatic coil spring. This spring will wind & unwind depending on the temperature. When cold the thermostat will hold the choke valve closed. As the temperature warms up, the spring expands and allows the choke valve to open.
Choke Vacuum Piston
The choke vacuum piston is linked to the choke butterfly by a small linkage. At idle, which will have full vacuum, the piston will be pulled into the piston well. This puts pressure against the thermostat coil trying to open the choke valve slightly.
Offset Choke Valve
Not all carburetors will have this air valve. Marvel Schebler & the Holley 1 barrel carburetors are a few examples that do. The air valve is placed offset on the choke valve. This keeps the choke from causing a too rich condition.
Most automatic chokes systems will use a stove pipe to heat up the thermostat coil. The pipe heats up, using the exhaust manifold and the heat is then pulled up to the thermostat using vacuum from the carburetor, which is fed by the intake manifold.
What Happens If The Choke Is On Too Long?
Once the engine is warmed up, the choke should be open all the way. If not then you will get black smoke from the tailpipe meaning too much fuel is in the system. The black smoke is gas not being burned.
Excessive choking will over time cause excess engine wear.
The engine may die at idle, purge while driving and probably hard starting when hot.