Small Engines (Lawn Mowers, etc.): briggs choke, engine vacuum, spring tension

Question
QUESTION: 3 1/2 briggs automatic choke doesn't open after starting.  It is not thermostatic from prior experience it is supposed to open by vacuum when the engine starts but this one closes back on the spring after manually opening it after starting.  The spring tension doesn't seem to be excessive compared to another one I have.  I am puzzled.  Any ideas?

ANSWER: I am not sure which carb you are referring to, the Vacu-jet, or the
Pulsa-jet, or another.  Does the engine not start and run correctly?

If not, what is happening?

Let me know, and also give me all of the engine's model numbers, so
we can be sure we are talking about the same carb,

Thanks,

Fish

---------- FOLLOW-UP ----------

QUESTION: The engine starts and will only continue to run correctly if I manually hold the choke open.  What makes the choke open if not engine vacuum once it starts if the choke  works correctly?

Answer
I am assuming you have the automatic choke carb that has a rod
made into/on the diaphragm, and that is regulated by a buildup of vacuum on the chamber on the bottom of the diaphragm, so there could be gunk blocking that vacuum passage, or a tear in the rubber, a leak
etc..
   Since this is an older motor, the diaphragm is likely stiff as a
board anyway, so going through the carb is a good idea, the diaphragm is only a few bucks.