Usually there's an overflow tube _and_ a drain tube. The drain tube is the one that should be closer to the bottom of the float bowl. The overflow tube should not have a screw near it. It's main purpose is to release gas if there's too much in the carburetor. Like Mitch said, it's most likely from a non-functioning float valve. The valve is not closing and letting too much gas into the carburetor. A thorough cleaning would probably help.