One other thing, if you take a propane torch and turn it on but do NOT light it, and then hold it near the air intake when the engine is running you will be substituting some fuel (propane) for some of the air, and thus making the engine run richer. And you will not have messed with any of the carb adjustments, so when you remove the extra source of fuel, you are back to the original settings.
Anywho, the point of this with respect to the problem you are having is:
1) if the extra fuel makes it run longer or even not stall at all, then you have a big clue that somehow it is getting a lean mixture
2) and if it does not make a difference then you have a big clue that the mixture is not part of the problem.