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I recently replaced my two throttle cables with the dual control throttle from McMaster and things seemed to be going swimmingly, engine was purring along and then just before my first flight with the new cable it developed a vibration at low rpm. I redid the idle set up and now it idles at 1800 but the engine quits. It shakes at 2800 or less, but with the choke on there's no vibration at all. I've weighed the floats, they are fine.

I'm trying to find someone local to help me with the pneumatic sync but I wanted to see if anyone would say "oh, it definitely sounds like....." so that we could hit the ground running rather than be two clueless people trying to work it out from the maintanance manuals. I do have a friend "who knows off road vehicles" so I'm hoping he can help.

I've only managed to put 9 hours on this engine, had so many minor issues that I'm sure if I had any knowledge of engines would be fixed in a heartbeat but I chose the Rotax because it was "install and go" and it has been anything but. Anyway, that's just my way of saying please excuse the novice question. 

Hopefully I can get this working right again, get my 40 hrs on my plane and start enjoying my build. Thanks as always for any suggestions.

Paul

 

  • Re: Yet another vibrating engine

    by » 3 months ago


    Paul --

    You do not indicate which airframe you are using.  For your background information, here is how the carbs are set-up on the RV-12 (912ULS) -- see Page 12-7:

    https://www.vansaircraft.com/service-information-and-revisions/mm-maintenance-manual-4/

    --  David


  • Re: Yet another vibrating engine

    by » 3 months ago


    Thanks David, I'll check that out - I have a Zenith 750 STOL


  • Re: Yet another vibrating engine

    by » 3 months ago


    If you don't have access to some vacuum gauges then you can get a rough idea of the carb synchronisation by measuring the distance from the throttle arm cable connection point and the bracket that holds the cable adjuster. Set the throttle in the cockpit to different positions, idle, half and full throttle and compare the measurements between the two sides. That will tell you if you have approximate synchronisation.

    Based on your symptoms I suspect that you would have to be a long way out for the engine to actually quit. I would be thinking more along the lines of a blockage in the main jet, the idle jet or the very small drillings in the main venturi just downstream of the butterfly valve.


  • Re: Yet another vibrating engine

    by » 3 months ago


    My money are the carbs are way out of sync. It could easily be the carb sync and there is no substitute for doing this procedure and should be done before any flight. Do not let a mechanic say he can sync or set up the carbs by ear or listening. If you ever just want to change the idle then gauges need to be used. 

    Since it is new you may have had a piece of debris that got into a carb. Since you looked at the floats you should have noticed if the carb bowls were clean. It is possible something got into the jets, but not real likely. Just a minor likely. :) 

    p.s.

    I would recommend gauges over the electronic sync devices. You can diagnose and see things with gauges you can't on the electronic device.


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


  • Re: Yet another vibrating engine

    by » 3 months ago


    “Oh, it definitely sounds like” ...an air leak somewhere in the induction system. But, if all pneumatic connections are secure and leak free, then step back, do the mechanical synch, then a pneumatic synch, per the Line Maintenance Manual.  


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