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Old 28 Dec 2005
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1st power loss underload, now running badly?


Currently having problems with my 88 R100GS as follows. Any help would be great as am currently in Venezuela and not near any bmw mechnics:

The first problem was a hiccup/loss of power while cruising i.e with the engine under load at approx 4100rpm, if I droped the revs to 4000rpm it went away. The first time it did this I checked the spark plugs and all looked fine (i.e. all 4 tan colour- the bike is twin plugged). Eventually I thought it was me getting low on fuel as it seemed to go away once I filled up. (On the same day I had a faulty float needle on LHS carb which also ended up needing changing).

The next day for the first hour the bike was fine and then again when hot and under load the same thing started to happen. If I stopped and let the bike cool down it would go again for 1/2 hr and then when it got hotter (or so it seemded) the problem started again.

That night I checked haynes and it suggested looking at the HT leads and spark plug caps. I found the spark plug cap on the LHS lower plug was split. I tried to repair it with epoxy resin and insulating tape. The next day again after about an hour of cruising the same thing happened. At this point I cleaned all the jets and float bowls in the carbs (and accidently took out the RHS needle jet - but put it back in with the carbs on the bike, then read in the motobins `tips` i.e. you need to invert the carb as it might bend the needle so took it out and took the carb off and refitted it). After all this still had the same problem.

That night found a replacement spark plug cap so swopped it over. Also checked the carb diaphragms and the fuel tank tap filters (and changed the fuel filters). On checking the plugs I found the LHS ones were fine but the RHS ones were black and sooty i.e. too rich.

Next day the top end power loss problem was gone (so maybe the cap was the problem) but now the bike sounds terrible when at idle (really uneven and poppy) and is not right power wise when pulling away (it seems to smooth out though at highr revs) and I cannot get rid of the sooty plugs on the RHS. Up until this point I had not touched anything to do with either of the carbs set/mixture.

Since then I have changed to RHS HT leads, checked the float levels on both carbs, tried to adjust the mixture screw on the RHS carb to get rid of the rich mixture but it appears to having little effect (both when at idle in terms of the revs changing or in the colour on the plugs). I have also taken the RHS carb apart, cleaned it and reassembled it as the bike handbook said a sympton of sooty black plugs could mean the `needle jet position needs checking` - what does that mean (it only has one position as far as I can see)? I thought maybe I had damaged the needle the first time I took the needle jet out but to the naked eye it looks fine. But still I appear to have the same problem.

The only thing I can think of doing now is swapping the plugs over to see if it is them at fault? Anybody ideas?

Also will it do the engine any harm driving with one cylinder correctly set up and the other very much over rich?

Thanks in advance.

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Old 28 Dec 2005
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It does sound like you have two seperate problems: one that you solved (cracked cap) and one that you may have created while fixing #1.

I think you have to re-check all the carb settings. One thing you didn't mention is the choke setting. The choke cable can get caught on at several points - the splitter under the tank, and at the carb itself. This could cause the problem you are talking about. This has happened to me on several occasions when diagnosing carb problems.

Generally running overly rich is not damaging to the engine, whereas running to lean can be. Still, it is never good to have your bike running out of sorts. Unless you have a known problem like a broken part and it is a matter of riding on to get to the part or place you can fix your bike, it would be advised to solve the problem now. I tend to think it is a simple problem and just a matter of chasing down what was has been changed on that side that is causing the rich mixture.

Good luck, I'm sure others will have some additional suggestions.
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Old 29 Dec 2005
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by the way, how are your fuel and engine oil consumptions ?
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Old 29 Dec 2005
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Also check your valve adjustment. If the exhaust valve tightens up it pushes the valve into the combustion chamber then you loose compression and the bike runs rough as only one cylinder is working. This gets worse when the bike is hot or at higher revs.If the adjustment is not maintained exhaust blowing by the valve seat can burn out the valve ..esp exhaust valve. Often it is a sign of valve recession.


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Old 30 Dec 2005
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I had a similar problem last year in Russia. The temperature was -12 centigrade and i was thinking perhaps it was somehow temperature related. I just rode slower for a while as i wasn't inclined to tinker around and get frostbitten fingers. In hindsight i wondered if it was a dodgy fuel problem?
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Old 31 Dec 2005
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I once had a similar problem. Whenever passing or going up a hill, runnung hard, one carb would run out of gas and i would loose all my power. I changed the needles, jets and needle-valves. Problem solved! I still get the occasional lhs. float sticking and gas on my boot. Maybe i´ll try the 2-part floats when i get back to Canada.
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Old 31 Dec 2005
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...BTW, the needles reach into your jets and controll the flow of fuel into the passing airflow. The top of the needle has a series of grooves which hold a circlip. I believe the setting is the second groove down, but check your book to make sure.These parts are all VERY precise! although may look fine,ive been told they should be replaced every 30k. miles or so for optimum eficiency.´Mine looked new when i replaced them, but the difference with the new ones is night and day. WARNING!! be prepared to empty your wallet
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Old 10 Jan 2006
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Thanks for all your advice.

I think I must have unwittingly mis-centered the needle when I fiddled the first time around, as having stripped the carb again and put it all back together the problem appears to have gone away...........
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Old 30 Jan 2006
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This response might be to late for your trip but none the less.
I am sure you will still have the problem as you have not fixed the right one yet.
At almost exactly 4000 rpm the taper on the main needles is supposed to start lifting out of the needle jets (ones directly above the main jets.)This meens that the needles and needle jets is now controlling the feul mixture up to full throttle.
If the mixture is to rich the bike will refuse to accelarate above 4000rpm. (a feeling of power loss.)
The real problem here is mostly worn or incorrect needle jets.
Most people make the mistake of replacing the needles without the needle jets when in fact the needle jets is wearing more rapid than the needles.
Hope this will help some folks out there as it took me months of fooling around to figure this one out.

[This message has been edited by gsworkshop (edited 30 January 2006).]
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Old 5 Mar 2006
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A BMW that looses power and refuses to rev above 3500/4000 is often because the diaphram is holed and not providing the vacuum to lift the needle.

It can be repaired sometimes by using some rubber glue from a tube tyre repair kit to glue a repair patch made out of a (rubber)condom. fuuny to repair a diaphram with a condom!

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