What is a reason my bike dies out?

Yamaha xj650 maxim 81. I has fresh gas it turns.. Runs and idles yet it seems it struggles but a mechanic said thats norm for a bike that old. My quandry is why does it run getwarm and then stop. No pops no mive sputter out pet is clean n up to snuff. No leaks only tjing is the throttle doesnt roll off on its own im going to try carb cleaner in the tank soon. Any ideas? Please only helpful suggestion... Besides bringing it to a mechanic. I thought possible over heat too since its the old air cooled.