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

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  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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    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

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    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?

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