One of the big problems from bikes of the 70s-80s that sat for a long period of time, was that the inside of the gasoline tank would rust and clog up the fuel system. I remember reading and buying a chemical that would paint your tank from the inside. Prep work included putting a bunch of BBs in the tank and swirling them around to clean the rust.

I also dealt with carb floats not sealing. Also, if one of the floats has a hole in it, that will cause the same problem. Or it may be---with the ethanol in the gasoline, the rubber seals have eaten away. Seen that, too.