Then it is technically an afterfire or preignition (timing advanced too much due a bad coil or partially sheared flywheel key).
Depends on how rusted the fuel tank is. I have seen a few that had internal clogging the carburetor tower that had to be manually cleaned. This internal trash could plugging the idle circuit once engine vacuum sucks up fuel. This not easy to do either as it requires removing the steel ball plugs. But first verify that the valve clearances are within specs. Also if you have a spare carburetor laying around give it a try to see you actually have a carburetor problem or not.
I have also seen a handful of coil electronic to go bonkers after slight heating as cause some strange things.
