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Thread: R100S Starting Problem

  1. #1

    R100S Starting Problem

    I have a 1977 R100S that won't start. The starter cranks strong, but the motor won't start. not even a hint of starting. She started right up after I installed the battery the other day. I cleaned the bike, went to go for a ride yesterday. She started fine, ran about a half minute then stalled. I tried to start again and she made popping noise, now nothing. Just cranks and cranks. I had a reputable shop install an electronic ignition last fall, but the bike started and ran fine until put away for the winter. I'm stumped.Any ideas here?
    1977 R100S, 1978R80/7,1973 R60/5
    1977 Triumph Bonneville
    1970 Triumph TR25W
    1970 Honda CT90

  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    Well, probably going to need to confirm - gas, compression, spark. What type of electronic ignition? Something could have changed. Also be sure the kill switch is not accidentally on.

    Gas -- do the float bowls have about an inch of fuel in the bottom.

    Compression -- pull the plugs (check them as well) and put your thumb over the hole...manually turn the engine...should feel pressure against your thumb

    Spark -- with the plugs out, put them back into the spark plug cap and bungee them to the engine fins...ensure that the threads of the plug are touching the engine. Then hit the starter...should see spark and it should be a bluish color, not orange/yellow. You could also manually turn the engine with the key in the on position (not started) and listen for the plugs to pop when the spark happens. What do you see in the timing window when you hear the spark? Should be somewhere near the S-mark.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Are the valves set correctly? Fuel flowing?

  4. #4
    Well, almost immediately after I posted it occurred to me to check the plugs. I must have left the choke on or something last time i rode the bike. The plugs were black and wet. I put a new set in and she fired right up. I'll take her out later and do a plug check. Needless to say I feel pretty stupid right now.
    1977 R100S, 1978R80/7,1973 R60/5
    1977 Triumph Bonneville
    1970 Triumph TR25W
    1970 Honda CT90

  5. #5
    Registered User STEVENRANKIN's Avatar
    Join Date
    Dec 2006
    Spencerport, NY


    Ah yes, the wonderfully designed choke lever on the /6 to 80 BMW gets another victim. I have the same bloody system on my 78 RS along with a kick starter lever. I always grab the end of the kick starter lever when attempting to turn off the choke while riding.

    Yep, you flooded it. Now, after you have ridden it, I will bet everything will be fine.

    Chokes in general are something I am not sorry to see have disappeared from motorcycles and cars.

    Glad the problem is solved, happy for you it was not a long expensive fix. Enjoy the riding. Cheers, St.

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