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Please help! I have a 912 ULS in a home-built kit Savannah. It is only 19 hours since first flight and all worked perfectly until 18 hours. I was flying along smoothly at 5000 rpm, opened the throttle to climb - and suddenly it ran rough and began vibrating. Back at 3000 rpm all was well, accelerate to 4500 and it happens each time. Now it drops revs and shudders after about 10 secs at full power - ok as soon as it drops back to 3500.
Everything seems ok (prop pitch equal, fuel filter clean, electric pump working, floats below 6 grams, carbs in synch, mags seem OK etc.). AlI I can find is both spark plugs in 4 were obviously sooty (and in 2 to a lesser extent).
Does anyone have any suggestions as to where I should look next?
Best wishes Dr John (Adelaide, Australia)
  • Re: Sudden Loss of power at 5200rpm

    by » 8 years ago


    If been thinking since I posted this. There was a tiny bit of something that looked like rubber in the bottom of the right float bowl. Is it possible for debris to be sucked up and block the float valve when the engine is fully revved - then drop down, freeing the valve, when the revs fall?

  • Re: Sudden Loss of power at 5200rpm

    by » 8 years ago


    Hi John,

    To answer your question, yes. It could easily be debris. I would pull both float bowls and look inside them before the next flight since you already found something. These are the symptoms of debris in the bowls. It could have been other things, but you already found a smoking gun so eliminate that possibility first.

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


  • Re: Sudden Loss of power at 5200rpm

    by » 8 years ago


    It does sound like something has clogged the high speed jets. Make sure the carburetor slides move up and down easily. If you found something that does not belong in the bowl I would check all jets now.

  • Re: Sudden Loss of power at 5200rpm

    by » 8 years ago


    Thanks a lot guys. I'll pull the carbs and do a full check.
    I had a closer look at the floats and one on the right (see image) has a small area about 2mm long of what looks like a shallow "ulcer" - and it weighs 0.3 grams more than the others. The tiny black flecks look like float material. They're the new floats, installed before first engine run with all due care. Is this a known issue?
    image.jpeg (You do not have access to download this file.)

  • Re: Sudden Loss of power at 5200rpm

    by » 8 years ago


    Hi John,

    3 grams is perfectly normal. leave it alone. Weight them as a pair and if they weigh over 7 grams then replace them. That little spot won't hurt anything. Just clean any little loose debris off and it's good to go. It has nothing to do with its buoyancy.

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


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