over filiing can cause it, but if its full and sits in the sun it can happen too. i suggest splitting the vent into two lines, easy to do, run both down to where the original one is.
2012 Cross Country(Imp blue)
de-badged,(self tinted) Klockwerx flare, passenger backrest, passenger touring floorboards, VictoryHID kit, Miller Mufflers short baffles,Lloyds air filter(from witchdr.)Lloyds timing wheel(6*), adjustable drop link, cut seat, removed cats, PCV w/at.
1978(2009) Honda cb750 chopper