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My 912 ULS is a 2001 vintage but only has 2 hours run time on it, all within the last month.  It initially started and ran smooth for the first 2 hours and then started leaking gas out of the vent lines and idling rough.  During that time I checked the fuel pressure while running and it varied between 6-8 psi (rising at higher rpm) with a steady reading (no spiking of the reading high or low).  As a result, I put a level 3 service kit into the carbs and that seemed to fix the vent leaks for a short while but now the engine has become harder to start and once it does start it shakes violently at idle and it seems that the shaking causes gas to leak out of the right side carb vent tube again (but not the left side).  The carb floats were not replaced because they all are floating at the level they should be at in both carbs.  When I pull the plugs and turn over the engine to see if there is spark across the plug gaps there is spark at each plug but it appears weaker on the #1 and #4 plugs.  As the engine turns over and I pull each spark plug slightly away from the engine block where it is temporarily grounded the stronger plugs will produce a longer spark but not the "weaker" appearing ones.  Is there something more I can do or check to discover what may be the cause of the rough idle?

  • Re: very rough idle

    by » 10 months ago


    I wouldn't being playing with the spark plugs like that.

    You didn't mention a carb sync being done or where the idle rpm is set at. If you haven't done either then do a carb sync and set the idle around 1700 - 1800. Did you set the float height with the brass armature when you did the carb overhaul?


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


  • Re: very rough idle

    by » 10 months ago


    Roger

    I haven't done a carb sync yet because it idles so rough, shaking so bad, and is leaking gas out of the right vent tube so that it barely idles at all.  When I overhauled the carbs I didn't mess with the float height on either float valve because it had previously run so nice and the brass was all in perfect shape with only 2 hours on it.  It didn't seem necessary.

    Is it harmful to the engine to check for spark the way I did?


  • Re: very rough idle

    by » 10 months ago


    You have not done any sync at all??  One should do at least a mechanical sync when the carbs are put back on.  Did you do that?  What are the positions of the idle-mixture screws?


  • Re: very rough idle

    by » 10 months ago


    Yes I did the mech sync and adjusted the air mixture screws all per the Rotax manuals.  I never got to the pneumatic sync but had set the idle at 1800-2000 rpm.  The rough idle appeared suddenly after 2 hours of relatively smooth operation so I'm expecting to find something broke or out of adjustment somewhere but not sure where to look next.


  • Re: very rough idle

    by » 10 months ago


    I would take the carbs off and have another good look at both of them.  You say the floats "all are floating at the level they should be", but what actual weight were they?

    It sounds like you are saying after rebuilding the carbs the engine ran smoothly, at idle only, for a total of two hours but yet you did not do a pneumatic sync during all that time.  Or am I misunderstanding?


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