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Rough at low RPMs

in 912 / 914 Technical Questions 9 years ago
While waiting for the new floats, I tried running the engine with fuel off to lower levels in the bowls. For about 30 seconds before the engine tries to quit, it gets really smooth and the RPMs go up about 300. If I turn the fuel back on quickly it gets rough again. Same thing every time. This engine has never idled smoothly. Been synced several times. Idle set at 1800. Plugs are always black and a little soot around exhaust pipe. Any thoughts?

  • Re: Rough at low RPMs

    by » 9 years ago

    Hi Richard,

    Is the rough running only down in the idle rpm range?

    Before you put the new floats in take the carbs off and check or rest the carb float arms to the proper height of 10.5mm measured from the carb edge up to the top of the float armature. Do it to both side of each carb to make sure one isn't out of whack (technical term ;) ) from each other. Make sure the mag drop is fairly close to each other and not a huge difference between each ignition.
    Black dry soot is normal at idle rpms and varies from cyl. to cyl. between engines.

    If it runs rough after the new floats are installed and the carb float armature is adjusted and if it is only in the idle rpm range then I would pull the carbs and flush every orifice in the carb with carb cleaning spray followed by so high pressure air.

    I would do this now since you have a rough running and you have to pull the carbs for the carb float armature adjustment anyway. Might as well do it and be done with it instead of having to pull the carbs a second time. Many times doing what looks like extra work now is the shortest way to fix a problem versus nickle and diming it over a week's worth of time.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Rough at low RPMs

    by » 9 years ago

    Hi Roger,
    Thanks for the reply. Before I start surgery, I should elaborate that the float change took place a month ago and that the fuel levels are equal in the bowls at the level I read was correct (was it 1/2 inch w/o floats?). Rough running has been the way with this engine since new. This isn't a problem that developed over time. My point was that when the fuel levels in the bowls gets low but before quitting, the engine purrs so smooth like I always thought it should run. After reading so many posts of rough running, I thought it was just the way it's supposed to be but it does affect my confidence in the engine.
    Is it possible that it's just way too rich?
      
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  • Re: Rough at low RPMs

    by » 9 years ago

    Hi Richard,

    I would still check the float armature height and blow the carbs out while I had them off.
    I really believe this is where your problem lies.

    The surgery doesn't hurt :) and takes me about 1.3 hrs. To do both and have it back up and running.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Rough at low RPMs

    by » 9 years ago

    Fair enough. I'll give it a go. Thanks!
      
    Report

  • Re: Rough at low RPMs

    by » 9 years ago

    Hi Richard

    It may sound a silly question, but are your slow running circuit jets clean and mixture screws set.

    Mark
      
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