BMW K bikes (Bricks)

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Platinum member
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Hello all,

The bike in question is a 1991 K100RS ABS.

Recently, I've been noticing my tach jump a bit during acceleration.  Now, this ONLY happens under heavy acceleration, 3/4 to full throttle.  The tachometer will jump 300-500 rpm at a time as it rises.  Under normal acceleration, however, this does not happen.  That makes me think this could be something else entirely.  BUT, when the tach starts jumping, the engine gets a little rough, which, put 2 and 2 together, points directly to the hall sensor if I'm not mistaken.  I sometimes notice a little bit of roughness under normal acceleration, but up to this point have written it off as the engine being cold, even though it still happened occasionally with the engine warm.

I have been inside the speedo / tach assembly to calibrate and change lights, but this was a long time ago and everything came out right with no problems.  I've also had a history of hall sensor replacements on this bike.  I believe the current hall sensor is the 3rd one I've put in, all bought used from eBay.

With all this in mind, why would the tach start skipping only under heavy acceleration.  If it is indeed the hall sensor, I would think the tach would be prone to skipping at any time, under any type of acceleration.

Thoughts? Thanks in advance!

1968 BSA Lightning
1991 BMW K100RS 16v with K1100LT fairing


was going to say that on the early 8 valve models the tach pics off one of the coils

but cant say that is correct for the 16 valve models

if so ....could be a wiring connection lead issue to the coils ? 

or a failing coil itself ??

cleanup around the coilpacks  sounds sensible ?

I wouldn't disregard a plugs fouling issue either , as you say it only happens under big loads and higher fuel consumption ?

an easy option might be to start at the plugs and change them ?

I hope someone with a 16 valve could confirm this

good luck

cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
1987 k100rs ########   "Red"  - (red sports rs TWB style )
1989 K100rt #009637   "Black Betty"  (naked rt ala Nigel , now sporting an rs main fairing )

Rick G

Rick G
Hi Haven't see you round for a while, you must have been out riding.
It is highly unlikely to be a hall sensor because they don't do any of the hard work just tell the coils when to fire.
Firstly I would put new plugs in if you haven't already done it. If that doesn't do the trick check out the plug caps and leads for white powder which is caused by bad connections, plug leads for worn insulation or cracks.
Pay particular attention to the coil for #2 & #3 as that coil is where the tacho signal comes from.
Also it would pay to check the ignition switch and give it a clean as they are a continuing source of problems.
Check the wiring and connectors it's amazing the things that can happen.
I recently fixed a Goldwing that wouldnt start if it was facing up hill and it turned out to be a bad connection on a plug that moved ever so slightly when the bike faced up hill and lost the connection yet moved forward so to contact when facing down hill.

"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama

Bikes 2004 K1200GT (crashed I lived it died) 1999 K1100 LT.

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