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-   -   R100GS acceleration problem in 4th and 5th (http://www.horizonsunlimited.com/hubb/bmw-tech/r100gs-acceleration-problem-4th-5th-58918)

Roboyobo 28 Aug 2011 21:06

R100GS acceleration problem in 4th and 5th
My R100GS has just started to misbehave when I try to accelerate fast after changing up to 4th or 5th. There's a delay of one or two seconds before I get a response. The engine isn't racing at all, just no acceleration.

Otherwise it seems to be running well, starting perfectly, getting 50mpg. I just did a long tour of Europe with no problems.

Any ideas?


dc lindberg 28 Aug 2011 23:35

Ignition trigger Unit, i.e. Hallsensor.

Behaves like that.

So does poor HT-leads and bad sparkplugs...

Could also be dammaged diaphragms in the carbs...

My personal experience is ITU. After a while it starts to missfire at higher revs under load, then just at higher revs then it starts dying at any revs... etc untill one day, it simply refuses to start...
I have 2-3 ITU's waiting to be repaired - can't get hold of the hallsensors in Sweden any more. The least expensive company that did supply was hallsensors.de (or hallgeber.de ?). Thank you for reminding me that I have to mend mine :)

Oh, there are another few reason as well - wrong jetting, but I expect that that is not an option. Poor compression, 6kg or there about can also have weird effects. Final drive, gearbox and shaft can also have these effects if wear have gone too far...

Roboyobo 28 Aug 2011 23:47

Thanks. Yes, all kinds of possibilities. But the electrics are all new. I did have a carb blockage at the last service. However, it seems fine in the lower gears. I also had water in the gearbox. But as its running fine most of the time, and perhaps is ok when warmed up, does't suggest gearbox. Clutch? Would oil leaking into it cause this?

Roboyobo 29 Aug 2011 00:06

It happens like this: i'm in 3rd, i ease off the throttle, pull in the clutch, change up, release clutch, pull back throttle. No acceleration, engine note stays as it is. A couple of seconds later it starts to rev higher and accelerate. No unusual vibrations, no odd sounds, no misfire.
I assume that if the problem were transmission, the revs would rise without acceleration?

Running perfectly in every other way.

dc lindberg 29 Aug 2011 10:14

Hm... that changes the understanding of the problem...

I can hardly belive it, but if someone have fiddled with the clutch wire installation and the leverage arm at the gearbox and gotten them all wrong - sure, clutch slippage.

Less common on these engines, but it happens - clutch slippaged due to oil leak.
- o-ring sealing in oil-pump cover gone bad; will cause this phenomena
- leaking crank-sealing will cause this problem (if you have the new type of crank sealing this is not likely to be a possbility though)
- leaking sealing on central shaft/axel in the gearbox (more common on the older gearboxes)

- you have traction. Shifting from 3rd to 4th will shift from horsepower to torque; if the clutch is not engaged fully you will suffer this kind of experence. When one shifts from revs to torque the demand of amount of friction between the two steelplates rises, and if the friction is not high enough it will result in slippage. The weak link is the clutch plate - when worn or exposed to oil, it will not be able to transfere the added demand of torque between the two steelplates - it will simply slipp.

- you do not state that the rev rises but speed remains; hence, it does not sound like a clutch problem.

The carbs:
- if you skipp shifting and allows for far higher revs, will the speed rise above the speeds where you get stuck now?

Delay respons, 1-2 seconds, is so immense that that does indicate an electric reason. However, I have suffered a similar problem once - carbs with floats in such a bad condiction that they did not float. I never figured out how they caused this accelleration problem - I simply changed them since I did not like the pool of petrol they left all over, incl getting my booths soaked in gas...

- what you are sure it can not be, may very well be it...
- what it obvious that is must bee, can just as well prove not to be it...
Meaning, a new item may fail. A mechanic may make errors. etc What you expect a fault to give rise to, may very well be a wrong expectation, i.e. that the fault gives rises to unexpected things.

Roboyobo 29 Aug 2011 18:52

Thanks Albert. The float was sticking at the last service, and I am getting petrol leaking out of that carb. Annoying, as I had the carbs refurbished a year ago (although not by a BMW specialist). Today it seemed fine. Although i'm getting a different symptom - when I stop and go into neutral, the engine revs too high, and then after a while settles back to its correct idle speed.

I do enjoy this airhead GS business! Even though its never straight forwards and certainly not cheap. I'll be taking it back to Andrew Sexton for a service soon - he's done some excellent work on it. After ten years of owning it and never getting it to start well, it now starts immediately as the button is pressed.

dc lindberg 30 Aug 2011 08:14

That kind of surge is caused by air-leaking.
Common situation with Bing CV.

This is the "ranking" of these causes:
No.1 is the rubber hoses
No.2 is the steel lids
No.3 is the choke gasket
No.4 are the other options (see below)

This is what to check:
- steel lid at top of carb dome; will leak. Need to be sealed.
- choke gasket. Replace and do "soak" the new paper gasket in grease before installation.
- rubber hose between carbs and heads dried and cracked; renew.
- stubbs in heads become loose; re-set. Use some gooey to seal the threads. I've found No.6 Form-A-Gasket to work nicely.
- butterfly valve not properly installed. Expect to need a new shaft and new screws - unlikely that you will be able to unscrew the two tiny screws and still have the threads intact in the shaft...
- mixture screw not at the correct position (amount of turns)

I may have forgotten one or two causes.

Roboyobo 29 Nov 2011 21:35

Solved. The seals between the crankcase, clutch and gearbox were all leaking, and the clutch was worn out. The symptoms were a bit confusing though, as there was no slippage in lower gears.

All fixed by Andrew Sexton, along with the carbs cleaned and balanced and ££££££ of other stuff. Now running well, although I have to get used to riding with a properly functioning clutch, which is a little harsh. I have a new friction plate, an oil resistant version from EBC.

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