Before you go too deep try properly adjusting the carb w/the engine warmed up to operating temp.

If you have a vacuum gauge, connect it to the manifold vacuum port and adjust the fuel/air mixture and idle speed to get the highest steadiest reading at idle.

IF the above doesn't correct it test for a vacuum leak at the carb, where it bolts to the manifold and where the manifold bolts to the head.
Assuming you have an automatic choke, your symptoms sound like a vacuum leak, i.e. as the choke opens the fuel/air mixture leans out and a vacuum leak will make the idle mixture too lean to idle.
Course when you don't know the history of a carb a rebuild is a wise move.
Good luck
Dave

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
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