If that doesn't fix the issue check the tip over valve, it may be stuck. It's a little plastic piece in the vent line clipped to the frame behind the carbs, also check the hose under the bike to be sure it isn't clogged with some mud or tar or something you may have kicked up. Most likely is pinched under the tank though.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!