The original hot air choke uses a metal spring to move the choke from closed to open position. Actually the thermostat is wound tight against the choke shaft closing the choke. Hot air is brought up from the exhaust manifold to the choke housing through a hot air tube. A small vacuum port feeds vacuum from the intake up through the carburetor and into the choke housing, which helps draw the hot air up through the hot air tube. As the choke thermostat heats up, the spring unwinds letting tension off of the choke shaft and the choke valve opens.
The choke piston is used to slightly open the choke valve when tromping on the gas while the engine is cold and the choke is closed. This piston does not use any seals, or O rings and needs to glide smoothly.
This video demonstrates how the choke system works and what it looks like assembled. This is a must see if you are trying to diagnose choke problems.
Fast Idle Cam Adjustment
Some of the Motorcraft 2150 carburetors have a plastic choke lever that at 1st glance may look like they cannot be replaced. This lever on the Motorcraft 2100 and early 2150 carburetors were simple to replace. The later 2150’s take a bit more work, but can still be replaced.
Fast Idle Cam Clearance Adjustment
This adjustment helps coordinate the throttle plate with the choke plate opening. This will allow a smoother, faster running engine during the cold engine period.
- This may not work on every 2100, 2 barrel carburetor, but should on most.
- Loosen the thermostat cap and turn the thermostat 90 degrees in the rich (closed) direction.
- Position the fast idle speed screw at the kickdown step of the fast idle cam.
- The kickdown step is identified by a ‘V’ stamped on the cam.
- The 351 engine makes use of a 2 piece fast idle lever to provide clearance between the idle lever and the intake manifold.
- The tang on the top lever will align with the ‘V’ mark on the cam.
- Check the clearance between the lower edge of the choke plate & the air horn wall. Specification may be different for each application. Refer to the instruction sheet included with the carburetor kit.
- To adjust, turn the fast idle cam adjusting screw.
- Clockwise to increase the gap.
- Counter clockwise to lessen the gap.
- Se the choke thermostat back to specification.
- Adjust the dashpot, idle speed and fuel mixture.
The choke pull down on the Motorcraft 2100 2 barrel carburetor is operated by vacuum. When the engine is off, the choke valve is completely closed. If the valve stays this way when started, the engine will not run due to lack of air.
When the engine is 1st started the vacuum is at the highest point and pulls the pull down, down all the way. This opens the choke valve enough to let some air in.
If the diaphram is bad the engine may die after starting and when cold.
Watch to see if the diaphragm pulls down when you 1st start the engine. If not, replace it. Also check the vacuum passage to make sure it isn’t blocked.
Adjusting the Pulldown Diaphragm