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  #1  
Old 6 Jan 2015
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R80 GS Miss fire
Hello there . New to this forum but not to BMW bikes . This is my problem . The bike ( 1994 BMW 80 GS) started to lose power to the point of not being able to drive forward and stalling . It would restart and rev ( with lots of throttle required ) but misfiring with no power . checked fuel and drained float bowls but just the same . A resorcefull AA man rigged up an twin out put coil and leads ( different plugs ) and then a different ignition amplifier all with no change . this pointed to the hall sensor so after recovery I fitted a new hall sensor .
The bike now starts easily and revs with normal throttle but has a slight misfire at idle to 1000 rpm but one cylinder drops out completly as the revs rise to full advance . I have fiitted a New coil and ignition amplifier and plugs . Using a strobe light showed that the Right hand pot was not firing at all above 1500 ( strobe not firing ) .
I reversed the input wires on coil and the left hand pot then started to miss in the same way ? ? The timing appears to be set correctly and advances to full advance .
there is a good supply to the coil .

Any ideas anyone ?
Last edited by gengisfx; 6 Jan 2015 at 21:53. Reason: spelling error
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  #2  
Old 6 Jan 2015
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Hello gengisfx and welcome,
I have had a misfire on my GS caused by one of the terminals in the block connector which goes onto the amplifier coming lose and not making contact properly. It could only be seen by looking into the block when removed and it appeared slightly further back than the others, also check the plug from the bean can as I have seen the same thing happen there.
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  #3  
Old 6 Jan 2015
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Thank you Mark,
I will check that out at the week end .
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  #4  
Old 6 Jan 2015
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As the hall effect switch, amplifier and coil are all common to both sides .. I suspect it is not them. I've be checking the spark plugs, spark plug caps, HT leads.
Of course the miss could be fuel related, or valves ..
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  #5  
Old 7 Jan 2015
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Thanks for the ideas Warin ,
I've changed the fuel and I'm thinking it must
Be electrical because the misfire changes side when I reverse
The coil input wires ( which makes me think ht leads
And plugs ok ) I can't think what would make the coil
Produce a missfire on one output ?
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  #6  
Old 8 Jan 2015
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You got the standard bmw set up? One coil?

When you say 'coil input' you mean the low voltage connections - one to ground, the other to the amplifier? Or do you mean the HT leads - thick wires leading to the plugs (these would be termed 'output' rather than 'input')?

Swapping the HT leads from one side to the other can isolate
  • the spark plug .. if swapped at the spark plug
  • the HT lead or spark plug cap .. if swapped at the coil .. and the above test OKed the spark plugs

IF it truly is the coil .. it would be unusual. No cracks on the coil? Easy test for cracks on the coil .. pour water over it. Or look at it working in the dark to see if there are any fire works around the coil.
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Old 8 Jan 2015
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Yes , totally standard electrics . I have fitted a new coil
With no change . HT leads and plugs swaped left to right with out the
Misfire changing sides . The only way to get the misfire to change sides
Is to swap the green wire and black wire into the
Coil .
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  #8  
Old 4 Jul 2015
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Coli problems R100 1993 GS
Hello

Currently have problems with my 1993 R100 GS.
And are also currently in Baku , Azerbaijan where are supposed to be only for three days and Leaving at the Astansa Iranian border tommrow.
So short on time to research and read everyones notes pratically on a small smart phone back in the hostel. and they don,t have a printer at this cyber cafe.

My right hand cyclinder spark plug is running very eractically at lower revs.
I have replaced my spark plugs and replaced right hand side spark plug lead with a new one.
Which improved things. but problem is returning.

Is there a soulution on the road to getting me too the border , ie matching coils from other vehicles?

Also I was sold a spark plug lead by a BMW shop with a different rating from the one I replaced.
The original was a 5k Ohm rated and the replacement is a 1K ohm rated.
What will this do in tthe short term and long term?
I will try to replace as soon as I find a place with a 5k Ohm one.

Problem only started Just after replacing the head set bearings yet cannot see any wiring problems from the re-assemby so think it is just badly timing in that part.
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  #9  
Old 4 Jul 2015
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I would also check the valve clearance on that side, a tight tappet can also cause a misfire and burn out valves.
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  #10  
Old 4 Jul 2015
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checked valves
Hello
On small smartphone currently.
Just checked valves.
Seemed tight so loser.
Then put original spark plug lead back on only.
No firing at all.
Put new spark lead back on and it fired up.
Put both leads back on and still eractic firing.
Will try other tests from thread.
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  #11  
Old 5 Jul 2015
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Does that model have the coils stuck up under the tank? If so fire her up in the dark and see if the coils are arcing - best with tank off using a bottle for fuel. A spray of silicone was the fix in its day until you could get a new coil.
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  #12  
Old 6 Jul 2015
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Swapping lead sides leading to swapping the fault side would suggest that it is not the hall effect (bean can) nor the amplifier.

So swap the leads around - left side to right side and right side to left side.

Ensure the connection of the spark plug lead to the coil is clean and firm.

The 1k and 5k 'problem'. The 'amplifier' is designed for the 5k .. using 1k puts a strain on the amplifier and may reduce its life.

The tight valve .. is that the exhaust? You may have a burnt valve...
Last edited by Warin; 6 Jul 2015 at 02:37.
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  #13  
Old 8 Jul 2015
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Hello
Seems currently that both spark plug leads failed within a few days of each other. Plus both valves appeared tight. A local bike shop in Baku replaced the spark plug caps. The old ones appeared black and burnt when torn apart. Done 800 km since are running fine now. The morning my bike was trucked to the bike shop nothing was firing then by the time it was rolled off the truck one cylinder was firing again.

Do the valves on 70s BMW get looser and later models get tighter? Ridden 70s bikes mostly.
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  #14  
Old 8 Jul 2015
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Quote:
Originally Posted by Kiwi-videos View Post
Do the valves on 70s BMW get looser and later models get tighter? Ridden 70s bikes mostly.
Heat. You are riding in a considerably hotter place than your use too? Add a few thou to the exhaust valve clearances. If you have problems starting .. it is probably the exhaust valves recessing ... I've had to adjust them every second day to get home. In hot climates .. add a bit of extra play to the exhaust valve (unless your water cooled).
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