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Revs jump sharply when accelerating

scottrichardwalker
scottrichardwalker
Barry Sheene
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Location : Newcastle-under-Lyme, Staffordshire

Solved Revs jump sharply when accelerating

Post by scottrichardwalker on Tue 02 Jul 2013, 10:36 am

Morning All

I went for a bit of a blast this weekend and noticed something happening with regards to the revs. When I opened the throttle the bike would begin to accelerate smoothly, then every so often, the revs would increase drastically up to around 8K then settle back down to a more "Normal" number again. 

Can anyone explain why this might be, if it is a "Serious" problem and how it might be overcome. 

Thanks

Scott


Last edited by scottrichardwalker on Tue 08 Sep 2015, 5:20 pm; edited 1 time in total
stormbringer
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the 900
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Solved Re: Revs jump sharply when accelerating

Post by stormbringer on Tue 02 Jul 2013, 11:55 am

@scottrichardwalker wrote:Morning All

<snip> then every so often, the revs would increase drastically up to around 8K then settle back down to a more "Normal" number again. 

Can anyone explain why this might be, if it is a "Serious" problem and how it might be overcome. 

Mine occasionally slips a gear, falls back in gear, slips again - and by then I've overcome the surprise and managed to pull the clutch, ease off the throttle and retry the gearshift.
Very annoying, as the engine goes 'off the chart'.

I believe this is what you see?

Don't think it's a serious problem. Others have reported gear slips like those I see.

Dunno how to remedy. Perhaps I just need to get better at gearchange. Actully I'm practising doing up-down changes while applying a little throttle - it seems to take away some of the klonking from the gearbox. Very different from changing gears in a Toyota Avensis (the kind of car that actively tries to bore you to death - it cushions you from all the good stuff), and it sure takes a little 'getting used to' operate an unsynchronized sequential gearbox.
scottrichardwalker
scottrichardwalker
Barry Sheene
Barry Sheene

Posts : 96
Location : Newcastle-under-Lyme, Staffordshire

Solved Re: Revs jump sharply when accelerating

Post by scottrichardwalker on Tue 02 Jul 2013, 6:38 pm

Hi Stormbringer

That is interesting, I hadn't considered that. I didn't think though that gears themselves could slip? Is it more likely that the clutch is slipping?

Has anyone else experienced a similar issue (Large increase in revs whilst trying to accelerate) and if so, how can this be remedied?

Thanks

Scott
teamster1975
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Solved Re: Revs jump sharply when accelerating

Post by teamster1975 on Tue 02 Jul 2013, 11:11 pm

It does sound like clutch slip to me but before you head down that road, how much free play have you got on the clutch lever at rest? Should be 2-3mm, if there's none then the clutch may not be fully released & causing you the slipping.
scottrichardwalker
scottrichardwalker
Barry Sheene
Barry Sheene

Posts : 96
Location : Newcastle-under-Lyme, Staffordshire

Solved Re: Revs jump sharply when accelerating

Post by scottrichardwalker on Wed 03 Jul 2013, 6:00 pm

Hi Teamster

When you say 2-3mm of free play, do you mean how much the clutch lever can be pulled in before it has any effect (Bites)? Sorry if that sounds like a stupid question. 

If I have understood that right, then I will check this tomorrow, but I don't recall it being "Tight", i.e. no free play. 

If it turns out to be a slipping clutch, what will it take to be fixed? Is it time consuming/expensive?

Any advice in this area would be greatly appreciated.

Thanks

Scott
stormbringer
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the 900
the 900

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Location : Aarhus, Denmark

Solved Re: Revs jump sharply when accelerating

Post by stormbringer on Thu 04 Jul 2013, 6:56 am

@scottrichardwalker wrote:If it turns out to be a slipping clutch, what will it take to be fixed? Is it time consuming/expensive?

A little bit of free play on the lever found on right engine cover is good, because this way the bearing inside will not be loaded. Which obviously prolongs its service life. Too much free play means you can't fully disengage the engine from gearbox, leading to gearbox abuse.
To fix a slipping clutch, you'll need to service it, meaning the right engine cover must come off. I don't think opening the engine without having a proper repair guide is wise. So if you contemplate doing it yourself, start by getting a Haynes manual for the CB. Here, you'll find a detailed instruction supported by pictures, tables with torque values, hints and whatnot. Then you can assess the task and determine if it's something you want to do. I've tried fixing a wet clutch on a Honda years ago (not a CB though), and I wouldn't say it is difficult. Just go slowly, use common sense and observe cleanliness.
You'll most likely have to get a new set of clutch plates, a side cover gasket, some new washers and possibly a new locking tab for the clutch unit. Also, since you remove the side cover in the process, changing engine oil and filter would be prudent - I personally don't like pouring half-spent oil back into the engine.

I read the section on gearchange mechanism in my Haynes yesterday evening to inform myself a bit. The CB uses a drum-mechanism seen as far back as late sixties; the half-assembled Dax-engine sitting on my workshop table has one similar to that, and there's not much that can go wrong. Also, the crank and valve drivetrain setup is awesome. Built to last, with matched components, massive bearings and a quality I've seen nowhere else. Trust-inspiring. But again and again the use of good quality oil and cleanliness while working on engine internals was stressed.

So if you decide to go ahead and repair your clutch, then make sure you have a dust-free and quiet place to work and set aside plenty of time and rags. And don't do anything until you've thoroughly read the relevant section in the Haynes book!

Happy tinkering Smile
eternally_troubled
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Solved Re: Revs jump sharply when accelerating

Post by eternally_troubled on Fri 05 Jul 2013, 12:22 pm

The 2-3 mm of freeplay is 2-3mm of *cable* slack (I believe)...   ie. a little bit of cable slack but not too much Wink
stormbringer
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the 900
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Location : Aarhus, Denmark

Solved Re: Revs jump sharply when accelerating

Post by stormbringer on Mon 08 Jul 2013, 9:48 am

@stormbringer wrote:Mine occasionally slips a gear, falls back in gear, slips again - and by then I've overcome the surprise and managed to pull the clutch, ease off the throttle and retry the gearshift.

Hehe - went riding - just for fun yesterday afternoon. 25 degrees C, clear sky, winding roads and holiday. Wonderful!

And then the bike did it again - slipped a gear amidst overtaking. Think I scared the poor driver, who had her window halfway down Embarassed 

Scared myself too. Nobody needs the powertrain to disconnect right then and there!
eternally_troubled
eternally_troubled
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Solved Re: Revs jump sharply when accelerating

Post by eternally_troubled on Wed 10 Jul 2013, 12:46 pm

It is possible to find a false 'neutral' while changing gear - it shouldn't happen, but sometimes does.  Often I find this kind of thing is to do with not operating the gear lever firmly enough, however it may be also related to gearbox wear if it is happening a lot.
gally
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Solved Re: Revs jump sharply when accelerating

Post by gally on Thu 08 Aug 2019, 9:52 pm

The owners handbook specifies 10-20 mm at the lever tip, i.e the ball end. This seems, in my 13,000 mile CB500, to translate to  2 or 3mm at cable end, Also at clutch case end around the same, maybe 3 to 5mm.
trevor machine
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Solved Re: Revs jump sharply when accelerating

Post by trevor machine on Fri 09 Aug 2019, 6:00 am

Just to repeat some points already made, this doesn't sound like a false neutral to me - because it seems to happen once a gear has been selected, and not during selection.

I've had two CB500s and put tens of thousands of miles on each of them. One was a frankenbike that was okay cosmetically but made up of mismatching bits and bobs (drum brake on a reg plate that should've been well into rear disc arrangement - with a swing arm cast for a hydraulic assembly, but then Nissin calliper on the front not Brembo, and an engine number that didn't match the logbook, etc. etc.). The other 500 I owned was bought with 8000 miles on it and looked brand new, was totally kosher, all standard, nothing tampered with apart from a Power Bronze screen fitted.

Guess which bike had a better feeling gear box, and never once found a false neutral? The beaten up bike.

The "new" bike will hit a false neutral at least once every thousand miles. It's horrible - but it doesn't alarm me like it used to. But while we're on the subject, this is absolutely why ROSPA, IAM etc. always say to select a gear in advance of overtaking, and thus to never change gear once such a manoeuvrer is underway. Of course, such advice and guidance has its roots in days (of British bikes) in times when sequential gear boxes weren't so well-engineered and were therefore more prone to false neutrals, etc. Needless to say, the last thing you want when getting passed a vehicle and you're far more likely to be in the path of oncoming traffic is to find you bike suddenly has no drive.

But, waffle aside, the OP's issue surely sounds more like a clutch problem? I've never known either of my bikes to slip out of gear once it's "connected". The grief has only ever occurred between cogs. I'm not ruling it out, but just reporting my own experience here.
Beresford
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Solved Re: Revs jump sharply when accelerating

Post by Beresford on Fri 09 Aug 2019, 8:21 am

This thread is nearly as old as me. 😉
trevor machine
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Solved Re: Revs jump sharply when accelerating

Post by trevor machine on Sat 10 Aug 2019, 2:04 am

shit yeah - Embarassed 

wonder how it panned out

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Solved Re: Revs jump sharply when accelerating

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