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Thread: 1992 K100 RS "breaks down / surges" at 4k - 5k RPM..

  1. #1
    Registered User Overkill's Avatar
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    1992 K100 RS "breaks down / surges" at 4k - 5k RPM..

    I have a 1992 K100 RS. It runs good at an idle and good up to 3,000 RPM. From 4,000 to 5,000 rpm it surges very noticeably. After 5000 it runs smooth again. Here is what I have done so far.

    Replaced all plugs and two wires (the wires were vibrating off over time)
    Replaced all vacuum caps
    Checked for air leaks, and checked vacuum hoses
    Synced the throttle bodies
    Good fresh fuel
    Valves were checked a few months (and very few miles ago) by Max BMW

    I am thinking it might be a weak coil, does that sound right?

    Thanks for you thoughts!
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    Curmudgeon At Large Bobmws's Avatar
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    Yet you didn't change the fuel filter as suggested to you in your earlier post...........
    Just keep flailing, it's good for the economy!
    Bob Weis
    '04 K12RS - Hannigan Hack
    www.earplugco.com

  3. #3
    Registered User Overkill's Avatar
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    I forgot to mention that the fuel pump and filter is new about a year ago when I bought the bike. I bought it not running with a bad fuel pump and it has done this since I got it. I would think if it was a restricted fuel filter it wouldn't smooth back out at 5,000 rpm and higher where it is requiring even more fuel. But I don't know.
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  4. #4
    Registered User Overkill's Avatar
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    Quote Originally Posted by Bobmws View Post
    Yet you didn't change the fuel filter as suggested to you in your earlier post...........
    Just keep flailing, it's good for the economy!

    I have not posted "earlier" on this bike so nothing has been suggested to me prior to your post.
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  5. #5
    MOA #24991 Pauls1150's Avatar
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    I don't think that a coil would be responsible for bad running within a very specific RPM - either they're bad all the time, intermittently at any RPM, or at high RPM. Possibly a "resonance" vibration causing an intermittent? Check the other plug wires for arcing in a dark garage; you can also put an inductive timing light on each wire to watch the action.
    WAG: the computer (fuel map) or a sensor providing input to it (TPS, airflow)?

  6. #6
    Registered User Overkill's Avatar
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    Quote Originally Posted by Pauls1150 View Post
    I don't think that a coil would be responsible for bad running within a very specific RPM - either they're bad all the time, intermittently at any RPM, or at high RPM. Possibly a "resonance" vibration causing an intermittent? Check the other plug wires for arcing in a dark garage; you can also put an inductive timing light on each wire to watch the action.
    WAG: the computer (fuel map) or a sensor providing input to it (TPS, airflow)?
    Good point, you are probably right. This may have to be a dealer resolved issue.
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    Curmudgeon At Large Bobmws's Avatar
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    Quote Originally Posted by Bobmws View Post
    Yet you didn't change the fuel filter as suggested to you in your earlier post...........
    Just keep flailing, it's good for the economy!
    Quote Originally Posted by Overkill View Post
    I have not posted "earlier" on this bike so nothing has been suggested to me prior to your post.
    Sorry, I read BCKriders post that you replied to, similar problem, and somehow your name stuck in my head as the original poster.
    Bob Weis
    '04 K12RS - Hannigan Hack
    www.earplugco.com

  8. #8
    Registered User Overkill's Avatar
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    Quote Originally Posted by Bobmws View Post
    Sorry, I read BCKriders post that you replied to, similar problem, and somehow your name stuck in my head as the original poster.
    No problem!
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  9. #9
    MOA #24991 Pauls1150's Avatar
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    Another thought - I'm not familiar with the K bikes' computers, but for the Oilhead brain, there is a procedure to "initially teach" the computer the relationship between throttle opening and TPS voltage (turn the key on but don't start the engine, twist the throttle slowly to full open and closed a couple of times, turn key off).
    Maybe this could apply here? Maybe a software glitch created a "dead spot" in the map?

  10. #10
    Registered User Overkill's Avatar
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    Update: Completed the following this morning.

    New fuel filter installed and new fresh gas.
    Adjusted TPS to .375 volts
    Cleaned the motronic connector

    None of these things made any difference.

    Went to put new heat sink compound on the ignition amplifier and mine is attached to the battery box without screws, it mounted on long studs. I got it off but I was removing the battery box and broke one of the rubber mounting studs so now I have to wait to get a new one to see if the new heat sink compound makes any difference.

    I don't really know where to go next.
    Last edited by Overkill; 10-06-2013 at 07:45 PM.
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  11. #11
    Registered User Overkill's Avatar
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    Update:
    Installed a new ignition module with new heat sink compound and still no change. Does anyone know what the specs are for the coolant temp sensor and the air temp sensor so I can test them?
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  12. #12
    Curmudgeon At Large Bobmws's Avatar
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    Quote Originally Posted by Overkill View Post
    Update:
    Installed a new ignition module with new heat sink compound and still no change. Does anyone know what the specs are for the coolant temp sensor and the air temp sensor so I can test them?
    Look here: http://www.ibmwr.org/ktech.shtml
    Bob Weis
    '04 K12RS - Hannigan Hack
    www.earplugco.com

  13. #13
    MOA #24991 Pauls1150's Avatar
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    In your original post #1, you state that the problem happens - repeatedly - within a very specific set of RPM.
    If this is true, then you need to specifically look at what happens within that RPM range "Only".
    Things like the ignition module or fuel delivery are probably not part of this equation.
    The only sensor - I think... - that could be a part of this might be the air flow.
    Otherwise I still lean toward the computer or something related closely to it; or less likely but possible, a resonance vibration causing something else to go intermittent.

  14. #14
    Dances With Sheep GREGFEELER's Avatar
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    For my friend it was the injector connectors

    A friend of mine developed what sounds like the identical problem with his '85 K100RS. It turned out to be a loose electrical connection on one of the fuel injectors. The male/female interface had loosed up, but after cleaning and tightening, his bike ran perfectly again. Hope this helps you.
    Greg Feeler
    BMW MOA Director & Ambassador
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  15. #15
    Registered User Overkill's Avatar
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    Quote Originally Posted by GREGFEELER View Post
    A friend of mine developed what sounds like the identical problem with his '85 K100RS. It turned out to be a loose electrical connection on one of the fuel injectors. The male/female interface had loosed up, but after cleaning and tightening, his bike ran perfectly again. Hope this helps you.
    Thank you! I will check that out!!
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