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Thread: 1988 K75C 166k troubleshoot spark issue

  1. #1
    PeteG
    Join Date
    Dec 2009
    Location
    Woodacre, California
    Posts
    105

    1988 K75C 166k troubleshoot spark issue

    I'm pretty sure this is a spark issue and would like to get input on the potential culprit.

    Background is that plugs were due for a change, still running pretty well. I still have trouble believing how reliable this MC is. In addition to changing plugs I also changed the cyl. head gaskets and bushings. Was getting slight seepage, I surmise was due to an encounter with a deer at 55 mph, raking down the left side of the MC.

    Condition of old plugs was excellent. Not too rich and not too lean. Nice tan/brown color. Gapped new plugs, a little never seize on the threads, hooked up wires (in correct position), added di-electric grease in the wire connector on the spark plug end. She fired up and sounded fine. Road test to freeway (3 blocks) had plenty of power. Entered freeway with plenty of power, ran about 5 mi. and upon exit was running VERY rough. Like it was running on two cyl. I made it back to home base but it clearly wasn't running properly.

    I'll pull the plugs and see which cylinder isn't firing. This will also tell me if wires were securely connected. Perhaps I have a bad plug? If I used too much di-electric grease could it have disabled the resistor gap on the wire, causing weak spark? I've used this grease liberally every time I've changed plugs. I don't know enough about the anatomy of the built in resistor in these wires to know if that could occur. If so, will something like "wire dryer" dissipate that grease or perhaps do any harm?

    The good news is that the seepage seems to have stopped at the cylinder head.

    In the interest of full disclosure I also washed the MC prior to changing plugs and gaskets. Nothing out of the ordinary happened here.

    As always, thanks in advance for any helpful hints in getting my ride back to life and me out of taking the cage for transport.

    Pete

  2. #2
    Rally Rat
    Join Date
    Jun 2012
    Location
    Olympia, WA
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    170
    I would start by removing all of the dielectric grease. It is non-conductive and can't do any good.

  3. #3
    PeteG
    Join Date
    Dec 2009
    Location
    Woodacre, California
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    Thank you Tim!

  4. #4
    A bozo on the bus deilenberger's Avatar
    Join Date
    Mar 2003
    Location
    Spring Lake NJ, USA
    Posts
    8,485
    There is no resistor in the K75 wires - they used a spark-gap instead to suppress the RF noise from the ignition. As was suggested - I'd remove the dielectric grease, and I'd also remove the antiseize. Bosch plugs are plated so they don't need antiseize. BMW spec's the plug heat range with a dry plug. The antiseize changes the heat path from the plug to the head - meaning it may run at a different temperature. The other possibility is some antiseize got into the cylinder and fouled one of the plugs.

    I'd remove all the plugs and carefully clean them. I'd also try to clean any residual antiseize from the plug threads and the head. Then reinstall the plugs dry- torqueing to spec.

    It's possible one of the wires also has failed - they are likely old enough and have enough miles on them. Unfortunately - there is no aftermarket direct replacement that I know of, and they're rather $$ from BMW... they also are impossible to really test since they'll read open-circuit even when perfectly good due to the spark-gap built into the cap.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

  5. #5
    PeteG
    Join Date
    Dec 2009
    Location
    Woodacre, California
    Posts
    105
    Thanks Don,

    I've had been working on it and had already checked off all of your suggestions.

    Turns out it was a pretty benign issue. I must be getting sloppy in my old age. One spark plug cap popped off.

    Everything is cleaned up and re-installed. My tired, but loyal, K75C purred smoothly on the test run . . .

    I think we can put this thread to bed!

    Thanks again,
    Pete

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