gas filler cap may be plugged or there might be dirt in the fuel line. If the cap is fouled, the engine will run until it pulls a vacuum in the tank and can no longer pull fuel into the carb. If you have fouling in the fuel line, the engine will run until it exhausts the fuel in the carb bowl. Poke the end of a paper clip in the small hole on the top of the filler cap, and try spraying little carb cleaner in the carb and into the gas tank. Luck, Kizmet