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Thread: 1994 R100R rough idle

  1. #1

    1994 R100R rough idle

    My 94 R100R suddenly started idling on one cylinder (it sounded exactly like my SR500 lol!). The left cylinder was not firing and bike would idle at ~700 RPM Rolling on the throttle and the left cylinder would start to fire and above 2000 RPM the bike runs perfectly.

    I screwed in the idle stop and the got the left cylinder to fire but the idle is rough and erratic. The bike has 14K miles. Any suggestions would be greatly appreciated!

  2. #2
    Registered User m_stock10506's Avatar
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    A complete baseline would be a good place to start. Valve adjustment/check, throttle sync and new spark plugs. Before you do that, check your throttle cables to make sure the ends are seated into the adjusters and there's nothing caught where the cable meets the pulley.
    Michael Stock, Trinity, NC
    R1100RT, R100, R60/6

  3. #3
    Registered User Guenther's Avatar
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    Spark plug wire broke/lose?

    /Guenther
    2017 F700GS

  4. #4
    Registered User STEVENRANKIN's Avatar
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    tune up mantra

    At 14K, have you ever adjusted the valves? Start there. Valve adjustment should be done about every 15K.

    It has electronic ignition and I would doubt the timing has changed. Besides, if the timing had changed it would screw up both sides not just one.

    A bad spark plug wire could be the problem. I would pull the spark plug lead off and check for any signs of shorting or weird looking discoloration in the spark plug boot. Then check it with an ohmmeter. I have forgotten the values for pass or fail sorry.

    If you have the valves set properly, and good spark, then it would be the carb. Problem could be anything from a bad/mis-adjusted float, to a booger in the idle jet.

    With the low miles, I cannot imagine anything has worn out in the carb.

    Good luck. St.

  5. #5

    Update

    I took the bike outside last night and when it was running the left exhaust pipe was glowing... too lean?

  6. #6
    Registered User STEVENRANKIN's Avatar
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    What have you found?

    So, what have you found so far?

  7. #7

    won't idle

    Quote Originally Posted by murty View Post
    My 94 R100R suddenly started idling on one cylinder (it sounded exactly like my SR500 lol!). The left cylinder was not firing and bike would idle at ~700 RPM Rolling on the throttle and the left cylinder would start to fire and above 2000 RPM the bike runs perfectly.

    I screwed in the idle stop and the got the left cylinder to fire but the idle is rough and erratic. The bike has 14K miles. Any suggestions would be greatly appreciated!
    My guess: if fiddling with the carb changes things, then it probably is the fuel delivery through that carb.
    I'd inspect the float bowl, float and float needle and blow some cleaner through the idle circuit.
    When the bike is running over 2000 rpm you're now on the main circuit so if its fine there then its something in the idle circuit.
    If that doesn't work then,although you're not suppose to do it for very long, I'd ground out the left plug at idle to see if you have a spark.
    Do a plug chop. When the bike is idling, shut off the ignition and immediately pull the left plug.
    if its sooty black its getting too much gas, if its white, not enough, if the plug is wet, then its not working.
    These things may help to determine if its carb or ignition and then go forward from there.

  8. #8
    Bluenoser
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    By now you've likely got it fixed but if not.

    When something happens out of the blue and you haven't done anything to cause it, then look at the simple stuff first. Check that the spark plug is connected properly. If it is then take the float bowl off and have a look in the bottom to see if you have any water droplets or dirt in the bottom. Either of those will get sucked up into a jet and cause your problem. When you took the bowl off was the fuel level about 1/2 inch below the lip of the bowl. While you have the bowl off, put a container under the carb. Turn the fuel on and work the float up and down carefully to see that the needle and seat are working properly and you have a good fuel flow.

    After you have checked the float bowl and needle and seat, then put the bowl back on, start the bike and see if that changed your issue. If not then you have to go deeper into the carbs and as well check the resistance on the plug wire of the affected cylinder. If you want, swap the plug wires and see if that moves the issue to the other cylinder. Then take the carbs off the mounts and then blow compressed air through all the jets and holes that you can find. You don't need a lot of pressure. At low speeds, it likely will be the idle circuit that is causing your problem and the small holes and passages are easily plugged, so concentrate on those first. It would be best if you had a carb kit, as you may have to replace the O ring on the pilot jet when you remove it. After this remount the carbs and start the bike and see if that has solved your issue. If so you are good to go, if not then you'll have to go further into the carbs.

    I have worked on bikes that have had similar issues and you immediately think of the carbs but in the one case it was a bad connection where the plug wire went into the plug. Screwed them apart and clipped off 1/4 of green corrosion so that there was new cable and screwed it back together. Cured the problem. So you need to work with a process of elimination from the simple to the harder.
    Last edited by cycleman2; 03-15-2020 at 05:39 PM. Reason: wrong word
    1995 R100Rt with Kenna Sidecar, 1986 K100RT

  9. #9

    Update 2

    Thanks for the help! Here's what I did so far:

    I replaced the idle jet and idle mixture screw on the left carb. Swapped spark plugs R to L. Visually inspected the L spark plug wire. No difference, left cylinder does not fire when idling but starts firing as soon as the throttle is advanced. Seems fine above 2000 rpm.

    Best,
    Paul

  10. #10
    Registered User STEVENRANKIN's Avatar
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    Other things

    Ok, you visually checked the spark plug wire and did not see any weird arcing or burning. Either test it with an ohm meter or, simply swap it with the other side.

    Did you check the valves for proper adjustment. At 15K they should be adjusted. A tight valve might cause poor idle.

    If the steps above don't solve the problem, you still have a problem in the carb. You may have a blocked by booger idle circuit. Are you sure the float is adjusted correctly?

    When you put the carb back together did you make the proper primary adjustment to the idle mixture screw? I don't remember just off had what it is but you can find it in a repair manual or online.

    Good luck. St.

  11. #11
    Registered User tanker4me's Avatar
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    My Airheads have had 2 instances of rough running as a result of resistor caps failing.
    The symptoms were different each time.
    1st time, the left cylinder stopped firing while stopped at a looong traffic light, I first checked that there wasn’t any water or junk in the float bowl, then swapped plug wires. Viola, the cap had failed.
    2nd time, there were intermittent “misses” when warming up, then the bike would run fine.
    After a bit of trouble shooting, I changed the caps and all was good after that.
    Or it could be...
    Another time, it was a small bit of the float bowl gasket that was pinched off that blocked the jet.
    Hope it helps.
    Bill
    Last edited by tanker4me; 03-19-2020 at 04:36 PM.
    We are all here for a spell, get all the good laughs you can.
    Will Rogers

  12. #12

    Update 3

    I swapped plug leads, no change - still not idling on L cylinder. I guess next step to check valve clearance, then go deeper into the L carb?

    Thanks,
    Paul

  13. #13
    Registered User Guenther's Avatar
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    still not idling on L cylinder
    When hot/cold/always? With or without enricher engaged?

    When starting cold an the motor is idling, does the left exhaust manifold warm up?

    Verify that the enrichers arms on both carbs move about the same when you turn the enricher lever on the handlebars.

    Next I would clean the idle jet.

    /Guenther
    2017 F700GS

  14. #14
    Hello,

    Yes hot or cold, it only idles on the Right cylinder. I replaced the idle jet and idle mixture screw.

    Thanks for your help!

    Best,
    Paul

  15. #15
    Registered User Guenther's Avatar
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    Any small twist on the throttle overrides the small amount of mixture from the idle system. If the motor runs fine besides idle I think there is something 'stuck' in the idle system.

    The air intake for the idle system is a small opening at the main air intake of the carb. I would blow compressed air into this hole. Also into the hole of the idle jet and the hole of idle mixture screw.

    /Guenther
    2017 F700GS

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