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Thread: R100 Suddenly Slow to Decelerate?

  1. #1
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    R100 Suddenly Slow to Decelerate?

    When riding the 93 R100R the other day I noticed when coming to a stop the engine idling fast then slowly dropping to maybe 1,100 rpm. (I usually set it at 1,000.) Continuing home I could notice the reduction in engine braking. Just curious what would cause this? It never did this before. Neither throttle cable was tight and it was actually pretty cool during the ride. Nothing in the way of adjustments were done prior to this ride. Throttle cables slide well and actually was replaced at about 12K. The bike has 25K now. Valves set and carbs synced about 2,000 miles ago.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  2. #2
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    Quote Originally Posted by milo View Post
    When riding the 93 R100R the other day I noticed when coming to a stop the engine idling fast then slowly dropping to maybe 1,100 rpm. (I usually set it at 1,000.) Continuing home I could notice the reduction in engine braking. Just curious what would cause this? It never did this before. Neither throttle cable was tight and it was actually pretty cool during the ride. Nothing in the way of adjustments were done prior to this ride. Throttle cables slide well and actually was replaced at about 12K. The bike has 25K now. Valves set and carbs synced about 2,000 miles ago.
    Quick, not quite random, thoughts. Induction air leak. Sticking advanced/retard. Sticking float(s). Carb diaphragm(s) starting to split. Check it that order for no reason other than easiest first.
    Mark

    Current - '74 TR5T : '93 R100R : '06 ST3s ~ Past - Variety of British, French, Italian, Czechoslovakian, and German bikes from the '70s, '80s and '90s

  3. #3
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    Thanks, Mark. Not sure how to check for the advance sticking but I will check the carb floats and diaphragm. And look for a leak.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

  4. #4
    Benchwrenching PGlaves's Avatar
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    Usually (with Airheads there are few absolutes) an increased idle or slow drop to idle is caused by one of two things.

    1. Sticking ignition advance mechanism
    2. An induction air leak

    Sometimes you can test for either as follows. Run the rpm up and see if it drops back. If it doesn't, then lightly load the engine by holding the brake and starting to let out and slip the clutch. This will lower the idle speed.

    If it stays down it is usually the ignition advance.

    If it goes right back up it is usually an air leak.

    You can observe the advance with a timing light. At full advance you should see the full advance (usually "Z") mark centered in the hole. When it drops back to idle you should see the normal idle advance hash marks and usually "S".

    Given your on-road symptoms I think the ignition advance is sticking. However, they might be caused by the ignition timing just being set too high. That is, it advances and retards correctly the both marks are past the hole.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  5. #5
    Registered User ebeeby's Avatar
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    Not sure on your model but on my /5 a return spring went missing as I rolled into Colorado Springs (on the left carb). That was my lesson in "check the easy things first".

  6. #6
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    Ebeeby, I'll double check that the springs are still there.

    And Paul I'll try your test running the rpm up then braking it back if necessary.

    Thank you both.
    14 R1200GSA, 93 R100R. No car is as fun to drive as any motorcycle is to ride.

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