After 15 years sitting your complete fuel system from tank to carbs almost certainly need a good clean out. Even if your tank isn't rusty inside and dropping rust particles and rusty dust into the fuel pipe, the old gas would have gummed everything up. Front to back pipe could be partially blocked too. Clean out or replace the fuel pump. Fit a new filter. If there is a lot of rust it can block after very few miles so have a few on hand. Rebuild the carbs, as in remove and disassemble and clean everything out. More gummy gas and rust very likely, hence you being able to move the jet and getting it running again for a while. Float valves also prone to sticking with old gas. Be methodical but be prepared to go through the whole system. Do a stage at a time. Tank, long pipe (airline helps) pump, filter, carbs. Good luck.