I got a 2003 cr 125 around 2 months or so ago. At first it just needed a new top end. After we put that in and started riding it, I noticed when I shifted into 4th gear it would rev like in neutral. It never ignites into gear. Just revs. Every other gear works 1-3 and 5th. Shifts smooth in all those. Does anyone know what it could possibly be causing this?