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Rpm Jump


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Hi there, I've got a question about my Moriwaki, which has a stock Honda CR250X motor in it. Recently I've noticed that just under 11,000 RPM, when I am accelerating, the engine sounds like it revs up suddenly and I see the tach jump up a few hunderd rpms, then continue its normal smooth increase. It ONLY does it right below 11,000 rpm. (Redline is around 14,000.)

 

At first I though maybe I was spinning up the rear wheel but it happens even with the bike straight up and always at the same rpm so I don't think that's the problem. I've only noticed it (so far) in 4th gear, not sure if it does it in other gears.

 

What could be causing this? Could the clutch be slipping, or some other transmission issue?

 

The power feels consistent through the sudden rev - there might be a very slight hesitation but definitely not a surge in power - it is just the sound and the tach movement that is catching my attention and making me wonder if there is a problem brewing.

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There is only two things that can cause rpm to jump rapidly; slipping or extra acceleration. If acceleration is constant, then something must give. AFAIK, it cannot be something in the gearbox, because it would jump, not just let the rpm down. Nor can it be the final drive chain because it's the same thing. So we're left with the clutch or the tyre. However, the tyre would be most likely to slip in the lower gears and the clutch in the higher gears. If engine torque is at its peak around 11k, it may be the clutch being just a bit too weak to withstand it in 4th gear and higher. If you do not notice it in 5th and 6th, it could be that due to the spacing being tighter that you do not let the rpm getting that low under acceleration in those gears.

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There is only two things that can cause rpm to jump rapidly; slipping or extra acceleration. If acceleration is constant, then something must give. AFAIK, it cannot be something in the gearbox, because it would jump, not just let the rpm down. Nor can it be the final drive chain because it's the same thing. So we're left with the clutch or the tyre. However, the tyre would be most likely to slip in the lower gears and the clutch in the higher gears. If engine torque is at its peak around 11k, it may be the clutch being just a bit too weak to withstand it in 4th gear and higher. If you do not notice it in 5th and 6th, it could be that due to the spacing being tighter that you do not let the rpm getting that low under acceleration in those gears.

 

I should clarify that it is a 5 speed bike, and 5th gear is a weak gear that doesn't pull as strongly as 4th - I don't use 5th much and don't shift into it until over 12K. So your answer makes a lot of sense, it is certainly possible that around 11K is the peak torque and the hardest work the clutch ever has to do is in 4th gear at that RPM.

 

So... currently the bike pulls strongly and my starts are good, so the problem may not be immediately obvious to a mechanic - is there a way to check the clutch to see if it is getting worn or weak, to try to confirm it as the source of the problem?

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You need to open the clutch and measure the length of the springs, thickness of the friciton plates and look for warpage on the steel discs as well as blueing. Any of these can cause a clutch to slip under load. Also, if it is slipping from wear, it will get worse since each slip - also from a standing start - will wear it more.

 

BTW, have you gone to a different oil lately or could fuel have entered the oil and thinned it? Such things can also affect the performance of a clutch even if the clutch is 100%.

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You got it right about the slip occuring at the torque peak in a high gear, next it will just start slipping at high revs. I wouldn't bother measuring or inspecting givin the amount of time you have on it alone! I would replace all the friction and pressure plates along with the springs. There will be some bigger pressure plates and you may need a few of those so you can set the pack hight with all the new parts. When you have it apart make sure the friction plate ears haven't beat grooves into the clutch hub teeth, that can cause the clutch to act funny.

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You got it right about the slip occuring at the torque peak in a high gear, next it will just start slipping at high revs. I wouldn't bother measuring or inspecting givin the amount of time you have on it alone! I would replace all the friction and pressure plates along with the springs. There will be some bigger pressure plates and you may need a few of those so you can set the pack hight with all the new parts. When you have it apart make sure the friction plate ears haven't beat grooves into the clutch hub teeth, that can cause the clutch to act funny.

 

Yeah, I was thinking about that - I do have quite a few hours (and a number of race starts) on that clutch now, and don't know how many it had on it before I got it.

 

Ok, thanks for the advice, I'll get it done before the next races.

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Did you have the oil changed recently? Some additives in (car!)-oils that are supposed to reduce friction are poison for motorcycle clutches.

 

Yes, it gets changed often because I am racing it, it was changed right before my race. It was done by a motorcycle mechanic, hopefully using the right oil, but I can check. What should I look for as a bad oil or additive for a race bike? This bike has a CR250X dirt bike engine.

 

Thanks for the tip about the oil as something to watch for.

 

I'll be changing the clutch anyway due to a high number of hard hours I've put on it, it's almost certainly due for it.

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Did you have the oil changed recently? Some additives in (car!)-oils that are supposed to reduce friction are poison for motorcycle clutches.

 

Yes, it gets changed often because I am racing it, it was changed right before my race. It was done by a motorcycle mechanic, hopefully using the right oil, but I can check. What should I look for as a bad oil or additive for a race bike? This bike has a CR250X dirt bike engine.

 

Thanks for the tip about the oil as something to watch for.

 

I'll be changing the clutch anyway due to a high number of hard hours I've put on it, it's almost certainly due for it.

Look out for additives of any kind, if in doubt use the pure mineral oil with no synthetic stuff in it . Motorcycle oils have special specification codes, I'm not sure if this is international, here in Germany the norm for motorcycle oils is "JASO MA" .

HTH

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