I have a Honda CBF125cc 2010 model. I have recently tightened the chain and changed the sprockets. In the last couple of weeks my bike seems to skip 4th gear. I can physically move the gear shifter into the correct position however, my bike will do one of two things: 1) it will sound as though it is still working in the 3rd gear and the engine is still struggling as I am going too fast for the gear. 2) it will go straight into 5th and the rev counter will go down really fast and the engine quiets down and sounds exactly like when your in a gear too high for your speed. When the second scenario happens, especially uphill, I struggle to go past 15-20mph, bearing in mind not long ago this bike was a perfect performer and raced uphill with no problem.
I was thinking that the Gear box may be worn or clutch could be slipping. Does anyone have any ideas what could be causing it or how to fix this sort of problem?