Sounds like the petcock may be plugged and likely a dirty carb .
Drain the fuel tank ,remove and clean the petcock . Be sure both reserve and On passages are clear . Re-install .
Remove and clean the carb . Remove the jets and be sure all orifices are clear . Brake cleaner does a very good job . Insure that all passages are clean /clear by witnessing flow of brake cleaner through every 'port' and that passages corresponding 'exit' point .
Everything inside the carb has to be pristine .
Wear eye protection when spraying brake cleaner . ;)