Watch a video about how this choke pull-down works:
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.