'77 R100RS coil failure?

Discussion in 'Airheads' started by fishkens, Sep 9, 2018.

  1. fishkens

    fishkens Long timer

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    Hi All,

    My '77 R100RS stopped running: points, oem coils, single plugged, essentially a stock ignition system.

    I departed Priest Lake, ID in the cool of the morning. Rode about two hours to Spokane and the day started warming up. On the ride it missed a few times at about 80 mph. I stopped in Spokane to visit a friend and get lunch and then headed west on Hwy 2. Temps rose to about 95 degrees and the miss became frequent within 30 miles. I pulled over in a shady spot and proceeded to clean all the connections. The point gap looked good, the plugs looked good, there was gas in the tank.

    With a bit of optimism I headed back on the road and the bike stopped running w/in a mile. The battery was strong as it gladly cranked as I looked for spark (orangish), swapped plug wires, etc. The starter would spin but it wouldn't fire.

    I walked the bike back to town and trailered it home (after a friendly ride).

    When I got home I unbuttoned things, cleaned some more connections but didn't feel I made any notable changes. The primary side on both coils measured w/in spec (1.3 ohms) and the secondary measured 6.7K ohms on one side and open, .6M ohms, on the other.

    I shot a bit starting fluid (gasp) into the carbs and it fired. I made sure the bowls filled with gas and it ran as good as always. I've ridden it about 50 miles in 65 degree temps and it's running great.

    Now when I measure the coils both are in or near spec (1.3 ohms and 6.7K ohms) on both primary and secondary tests. User error on the first test is possible but I must have tried a half dozen times. Spark plug wires measure 5K ohms. Spark plugs haven't changed from run/won't-run/run. Likewise with the battery.

    1) Is it possible that one coil failed in the heat and kept both cylinders from firing a spark strong enough to run it, even on one cylinder? I don't understand the symptoms of single coil failure.

    2) Could a bad ground cable be the cause?

    I'm stumped as to what may have kept it from running. I haven't yet checked valve clearances but I'm guessing they're okay...guessing.

    Thanks for any ideas and suggestions as to what kept it from running or diagnostic tests I can run to keep it running.
    #1
  2. Piggdekk

    Piggdekk love speed, hate rush

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    when my coils where about to die (R100S) the bike wouldn't start hot, but it would restart once cold. After this happened a 3-4 times the bike died completely and I was not able to start it at all. So it is possible that when it cools down the coil measures fine. However, when the engine was hot the bike would misfire a bit (1-2 km), then backfire a couple of time (REALLY loud) and eventually stop.
    If your coils measured badly and are old I would just swap them, but FWIW Your symptoms seems a bit different than mine. You could try to heat up Your coils with an hair dryer and see if the resistance changes.
    luca
    #2
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  3. Warin

    Warin Retired

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    Reliable running into the future?
    I'd replace the coils .. and carry a spare condenser (that is the other component likely to be heat effected).

    Heating with a hair dry may help you find the problem/s. But that 0.6M reading is a clear indicator.
    #3
  4. fishkens

    fishkens Long timer

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    Makes sense. Will do. Thanks.
    #4
  5. oldgsbob

    oldgsbob Adventurer Supporter

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    the coils are wired in series so if one coil goes open circuit then no spark on either.
    Change the coils they must be ancient.
    #5
  6. 190e

    190e Long timer

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    If you do a hot resistance check then bear in mind the resistance values will increase perhaps as much as 10% due to the temperature coefficient of copper but you are looking for a much bigger change than that.
    #6
  7. Steve W.

    Steve W. Boxer Pilot

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    Condenser !! First!! I've seen them fail with heat, and just go flakey. Hard to check with a meter unless it just shorts out completely. Can be replaced with one for a old car (vw) with a wiring change, if you are on the road at a flaps. Those old 6v og bosch coils are bullet proof, if not corroded at the plug.
    #7
  8. spo123

    spo123 Man About Town

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    Do not forget that the spark plug CAPS can and often do fail.
    Easy to replace with a fresh pair of the NGK caps......either 5k ohm (electronic ignition) OR 1k ohm (points).
    #8
  9. fishkens

    fishkens Long timer

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    Thanks all. I've heated the coils with a heat gun until they measured over 100* F after a few minutes of no heat gun (thinking the heat permeated the coil) but still can't get it to fail. I had bypassed the condenser when it wouldn't start so I ruled it out. I do suspect the coils and will be ready to replace them.
    #9
  10. Piggdekk

    Piggdekk love speed, hate rush

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    I remember a distinct smell of fried wire when my coils gave up. In fact I thought something was burning, but then the first times it happened it restarted after a while, then it was dead.
    If they are old just change them and move on, worst case You've changed something that would have failed sooner or later.
    #10
  11. bmwrench

    bmwrench Long timer

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    Have you checked the wire that connects the two coils?
    #11
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  12. fishkens

    fishkens Long timer

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    I've brushed the connections and added dielectric grease to the spades and/or lugs. I haven't checked the wire for continuity or weaknesses. I guess a failure there would kill the spark. I'll look more closely. Thanks.
    #12
  13. Disston

    Disston ShadeTreeExpert

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    That small wire between the coils does fail occasionally. Seems more often that the coils failing maybe.

    I ended up with another set of coils years ago so now any time I suspect the coils I can put the other set on. No change so I still have 2 pair of working coils.
    #13