Doubtful that's the problem as long as it's not leaking. Once you are running, there is positive pressure in your tank. So if the screw is leaking, then fuel will come out. From your description, I'm just about sure that your fuel is running back down your fuel line during the prime, not letting fuel into your engine. I have an Ultra Stick that does that because the nose sits up on the stand. It takes about 15 seconds for the starter to pull fuel up through the line to get it going normally and hand priming nearly impossible. But if I tip the nose down to level, it primes well and starts in a second or two. Tank position plays a huge role in how easy your engine is to start.