Check the hose going to it is not blocked or restricted and make sure the connection on the carby end is clear. Although having said that the compression of a motor should be enough to force it shut. I'm assuming you're talking about the one on the 125 you're restoring with the G3 carb. Also make sure the fitting on the manifold is not loose and sucking air from around there. If its leaking it wont help your situation. Air likes to take the path of least resistance.
