I think he's talking about the drain tube, not the overflow tube. The screw is suppose to cut the drain off and when you loosen it, the fuel is supposed to drain out the drain tube. I had one that leaked like that before. It didn't matter how tight you tightened the screw, it would still leak. It didn't leak that bad, so I never fixed it. I just turned the gas off on the tank when I wasn't riding it. I really don't know how to fix this, other than getting a new bowl. Silicone or something around the screw might would work if the gas wouldn't dissolve it or if it didn't break loose on the inside and stop your carb up. The last thing you want on the inside of a carb is silicone.