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?

2 Answers 2


Is it possible you bent (or misaligned) the gear lever while changing the front sprocket? Maybe your lever is in contact with your engine case and it lever simply sticks to the case when you are upshifting. I can't think of anything else that a sprocket change would do. Also, if you change your sprockets it is a good idea to change your chain as well. A worn chain will eat your sprocket much as faster. The vice versa is also true. Therefore it is usually suggested to change them simultaneously.

  • Yes I changed both the chain and sprockets at the same time but have gone and tightened the chain once more since then. The bike was driving perfectly fine after the sprockets were replaced (which was about 4 months ago). The chain was tightened again on 10th Jan 2019(less than a week ago) and the problems started appearing and being obvious 2 days or so after this. Jan 14, 2019 at 11:43
  • I would also like to add that I took my air filter out to check whether it was the filter not letting enough air into the carburettor. It seemed to have improved the problem slightly but not by a lot. DO you think there could be something wrong with the throttle position sensor... I say this because when trying to accelerate by giving it more gas, the RPMS don't really increase at all but my engine sounds super load as if im going too fast for the gear, yet I am definitely not. Jan 14, 2019 at 11:48

Sounds like a slipping clutch, especially that it revs high then jumps back down to low revs (as the clutch eventually bites)

First, have you made sure you have enough slack in the chain, and it's not over-tight? This isn't to do with the clutch, but just checking :-)

Also, check there is slack in the clutch cable (if it is a cable) when the clutch is released. If the cable is tight, it needs adjusting. It doesn't need loads of slack, but just enough to wiggle.

A slipping clutch will result in the revs going up but not the speed, and is worse in higher gears and up hill.

A diagnostic for a clutch is to face the bike tight into a wall, and try to pull away. The bike should stall, even at relatively high revs (at least 5k for a 125). If you can let the clutch out a long way without stalling, you probably need to adjust or replace. Try adjusting first.

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