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Thread: R100RT Running Roughly

  1. #16
    Here is the update. Today we started going down the list and checked/found the following.

    1. Gas cap off to check for plugged vent: No change.

    2. No evidence of contamination in float bowls, gas drained from tank, or in old fuel filters or the new ones installed with the new fuel lines.

    3. Strong flow from both petcocks in the on and the reserve settings.

    4. Floats both appear to function normally.

    5. Oil is clean, at correct level and smells normal.

    6. Carb bowels are full immediately after rough running/stalling.

    7. Both mufflers are clean and clear.

    8. Left petcock leaks intermittently. It only happened once but when it did it flowed as though it was completely open and could not be turned off. I'm assuming it was leaking all winter and that explains the missing gas.

    9. The rough running and stalling occurs only at lower RPM's. It is consistent. Upon first starting (choke on) it starts and runs normally and will idle. Once it starts to warm up it runs roughly from idle up to about 2800 RPM. Once above 3000 RPM it smooths out. No hesitation like at lower speeds and no missing.

    A casual internet search came up with petcock rebuild kits for years up to 1980. Tomorrow I'll look into it more. Any help in this direction is appreciated.

    Early next week we should get a chance to take a look at the valve adjustment and a few other things. If today's details point in any particular direction we'll look into those as well.

  2. #17
    Day Dreaming ... happy wanderer's Avatar
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    Had a similar issue last fall and it turned out to be oil deposits in the main jet area and the idle circuit. You might want to try removing the floats and needle valve. Then you can remove the main jet parts and then remove the air mixture screw. Spray those orifices out with carb cleaner and blow (gently!) some compressed air through those passages if you have air. Make sure they are all clear and the idle screw is nice and clean. This chart shows what part of the carb is used at varying throttle openings. In your case you are having issues with the Air Screw slow jet and maybe just when the needle starts moving up the main jet.

    jet-chart.jpg
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  3. #18
    Registered User Guenther's Avatar
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    jhorn,

    thanks for answering all the questions in your complete update!

    My next focus would be the idle jet. Here's why: With enricher(=choke) on, gasoline is funneled through separate openings in the carb straight to the cylinder side of the throttle plate. At low fuel consumption (=idle) it dominates over the gasoline from the idle jet and the main jet. Once the enricher is closed the idling motor depends on gasoline coming from the idle jet.

    But this has nothing to do with the occasional misfires. It still does misfire, right? At which RPMs?

    How long ago were the carbs checked? Diaphragm, jets, o-rings, gaskets...

    /Guenther
    1989 R100GS, 2017 F700GS

  4. #19
    Vintage User nevada72's Avatar
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    As someone who just resurrected an 83 R100RT (among a lot of other vintage bikes), I can tell you the carbs will for sure need attention after sitting so long. My bike sat for 3 or 4 years and the carbs were in bad shape. Jets all clogged, o-rings fractured, and the diaphragms were toast. And it was put up dry.

    I would (and did) replace all soft parts, including floats. Once the valves (much more critical than I ever imagined) were adjusted the bike started right up. Now I need to fine tune the timing and balance the carbs. Oh....and the rear master cylinder needed a rebuild. Had nothing to do with the bike running, but having brakes is nice. Sitting long term does nothing for rubber in contact with fluids.
    1971 R75/5 - 1983 R100RT

  5. #20
    MOA #24991 Pauls1150's Avatar
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    After you get the carbs and the rest of the "typical" stuff gone through - there are several symptoms here that may also indicate a vacuum leak. Note that a small vacuum leak will have less effect as RPM increases.
    Examine the coupling tubes and clamps between the carbs and heads, and if your carbs have vacuum measurement ports, verify that their screws (or caps) are in great shape and secure.
    If your bike has the old vacuum "crossover/balance" lines, rip them off & cap the ports closed forever; while some claim no effect, they are another place where a tiny crack or pinhole in the hose will cause issues.

  6. #21
    Quote Originally Posted by jhorn View Post
    Left petcock leaks intermittently. It only happened once but when it did it flowed as though it was completely open and could not be turned off. I'm assuming it was leaking all winter and that explains the missing gas..

    A casual internet search came up with petcock rebuild kits for years up to 1980. Tomorrow I'll look into it more. Any help in this direction is appreciated.

    While it's not "the Airhead Way", I wouldn't bother rebuilding fuel petcocks. Readily available, and too critical and too cheap ($60 each for new Karcomas, per the MAX site) to dink around with, IMHO.

  7. #22
    MOA #24991 Pauls1150's Avatar
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    Just another WAG, with the caveat that everything else (as above) has been examined:
    Is it possible that the centrifugal ignition advance assembly is intermittently hanging up (sticking in place)?

  8. #23
    Moving on to the next steps.
    I found petcocks locally so Iíll pick them up this week. Iíll probably get both sides.
    Tomorrow we should be working on valves and carbs. Carbs have never been worked on under my ownership and not sure of before that.
    Any preferences for carb cleaner? Or any to avoid?
    This will be the first time working on carbs for me aside from checking the float bowls so any words of wisdom or helpful hints are appreciated.

  9. #24
    Valve adjustment checked out. Got too late to look at carbs Monday and we wanted to find a better diagram. Symptoms still the same after valve check. I picked up both petcocks. Will continue down the list next time schedule and weather both cooperate.

  10. #25
    John D'oh
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    Have fun riding it...

    Finding the source of the missing fuel was important. I had a half tank of fuel wind up in the crank case by the way which is how I know that one should never trust a petcock. There is good indication in your text that the idle jet / idle circuit are to blame. Giving your carbs a bath should help. Hopefully that does the trick. Diagnosing a problem by keyboard I can almost hear the "idle and sputter". 'Sputter and cough'. Silence ~ Give me video chat with a motor.
    John D'oh

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