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Thread: Sudden, brief loss of power - 1987 K75C

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  1. #1
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    Sudden, brief loss of power - 1987 K75C

    Symptoms: When moving at speed (happened anywhere between 15-50 mph), sudden (VERY) loss of power, with no response to throttle. Bike slows, of course, but engine somewhat curiously appears to remain "on." No odd behavior of dash lights. After a few seconds, power and throttle response return. Occurred about 6-7 times in ~60 miles of riding, but "clumped" into three instances, where it happened once or twice a few seconds apart. (27-mile each way commute, with a little midday driving in between.)

    My first guesses were: Fuel delivery problem. Suddenness of onset and resumption of normal response suggests electrical cause. Or maybe water in gas? That pernicious fuel-pump connector under the tank? Of course also suspect fuel filter/hoses/pump. Absence of funkiness with dash lights (and a much less catastrophic-feeling failure) suggests it's not a recurrence of my previously jiggly kill switch.

    Testing today found a faulty throttle position switch, but not in a way that could obviously cause the observed behavior. The switch tests in the "open" condition. That is, on a normal switch (or so says the one on my parts bike), the outboard pin and the center pin should be closed when the throttle is closed, and open when the throttle is open. (The center and inboard connector should be closed at WOT.) The presumed-faulty switch tests open regardless of throttle position.

    So, this shouldn't cause what I'm seeing ... or could it? We know some people remove this switch, which would leave the bike in the position I found in testing: circuit permanently open. If, however, the switch's internals are loose, such that the contacting bits could jiggle into a closed position, it seems to me it could cause what I experienced. Can anyone confirm an occurrence of this? Is anyone familiar with the internals of this switch? I'll pull it apart tomorrow; just ran out of daylight today.

    Today's diagnostics also included:
    - Check battery terminal connections: fine
    - Check ground to frame: fine
    - Check spark plug & fuel injector connections: snug
    - Computer connector: snug, contacts clean
    - Pull & inspect spark plugs: fine
    - FPR vacuum hose: lookin crusty, replace next week
    - Crankcase vent hose: also crusty; order & replace
    - Check & reseat tank connector: Looks fine
    - Check for spark: Fine on all three zylinders
    - Check fuel pressure: 36-37psi (stationary, of course)
    - Drain tank; no evidence of water or crud in gas (vibration damper newish; < 10k miles)

    In progress, to be finished on return of daylight:
    - Replace fuel filter
    - Replace in-tank fuel hoses with appropriate new OEM hoses
    - Replace fuel supply hose (tank to fuel rail)
    - Check that tank vent pipe is clear

    On receipt of parts order:
    - Replace crankcase vent hose
    - Replace FPR vacuum hose
    - Replace fuel return hoses

    (Edited to add other diagnostics performed.)

  2. #2
    rabid reader dbrick's Avatar
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    Your description of the bike's stopping as "more catastrophic" when the kill switch failed suggests to me an electrical connection problem that causes the fuel pump to stop pumping. If the kill switch is turned, the lack of ignition causes the motor to stop pretty abruptly, while I think the motor's running-to-not transition because of the fuel pump stopping would be a bit smoother as pressure in the fuel rail declined.

    I would reexamine the tank connector and the wiring, as well as any other connectors in that part of the wiring harness.
    David Brick
    Santa Cruz CA
    2007 R1200R

  3. #3
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    David, thanks. Fuel pump power and that very connector were my first thoughts as well. It's a terrible design. I've reseated it as tightly as is possible, and replacing it with something that connects more positively is in order as well. In the meantime, I've also marked the wire positions so I'll be able to see if any work themselves loose from their current, tight position. The in-tank connections were very tight; no problem there.

    If the problem does recur, I will rig a way to continuously monitor fuel pump power, either via the fuse or something around the tank connector. What's difficult here is that I can't reproduce the issue on demand, and when it does occur it's so brief (and hazardous) that there's no way to stop for diagnostics. Or maybe I ought to rig that up now, though I don't want to introduce anything potentially problematic, particularly in the area of that connector.

  4. #4
    rabid reader dbrick's Avatar
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    James,

    I have a vague memory of reading about a K-bike tank connector failure within the last few months, but I can't remember if it was here or on the IBMWR mailing list. Edit: It was Scott's experience, mentioned below in Post #6.

    How about tapping the fuel pump and ground wires to also run a 12v relay with a pair of NC contacts; the contacts would be held open when the fuel pump circuit has power, and would close if the power fails. Wire the relay contacts to a warning light up on the dash. If the power fails (at least as far up-circuit as you can make the connections), the warning light will illuminate. You might see it even in the middle of your where'd- the-power-go? full attention mode.
    Last edited by dbrick; 01-21-2013 at 05:02 AM.
    David Brick
    Santa Cruz CA
    2007 R1200R

  5. #5
    Registered User Beemer01's Avatar
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    Classic symptoms of the under tank plug problem

    So I had an 85, but unless the Germans dramatically redesigned that 4 pin connector they are problematic and reseating them hard accomplishes nothing permanent. This problem has been well covered here by people smarter than me, but the core design of the male tank plugs and the female 'tube' connectors in the wiring harness is at the root of your problems IMHO.

  6. #6
    Still plays with trains. tinytrains's Avatar
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    It is not just the connector that goes bad, but the pass through for the wires. It is part of the float assembly. My K75 would just shut off after 15 minutes and would not restart for 10. There is a solder connection inside the float assembly block with wires soldered on both sides, when it got hot, the fatigued solder joint would open up. Had to replace the float assembly. This may or may not be your problem.

    Once I was sure it was a fuel problem, I ran a pair of wires from my fuel pump out the filler cap vent to a volt meter. The rode around until it quit and verified the voltage had gone away. Should you try this, make sure you a have good connection (as in lugs) bolted to the pump. You do not want any sparking in the gas tank! And don't ride with the fuel cap open.
    1988 K75 Low Seat
    2009 R1200R Roadster

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