Fuel will flow continuously in the "PRI" position, but shouldn't flow long in the "ON" with the engine not running. Have you checked the muffler for clogging? Take it off and give it a good blow(job). Sorry, I couldn't resist. But, you should be able to see if there is an obstruction in there. Air should flow freely through the unit. The carb must be perfectly clean with every single tiny hole properly open or it will act up. Tiny wire and compressed air should be used along with the solvent to make sure you've done a thorough job. I had to do one of mine three times before I got it right. Blah, blah, blah...I've talked too much.