72 250 stalling old fuel gum?
I had a 72 250/8, and loved it. I suspect what you have is some clear varnish from the old fuel intermittently plugging up the carb jets. Take them apart again and soak the carbs and jets in carb cleaner (xylene, tolueme,'aromatic solvents', paint remover .
If I remember right, the fuel pump is a mechanical one, and it so replace it the diaphragm will be bad, and the check valves full of varnish just waiting to plug the jets up again.