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My Rotax 912 ULS is in a RAN-6 amphib. 750 hours on engine, carbs synced about 25 hours ago. I burn primarily 91 octane non-oxy, but sometimes add 100LL. Have 50 hours on new spark plugs that were re-gapped according to rotax recommenation. It runs like a top the vast majority of the time, like months at a time (50 hours) for. However, every once in a while, the engine "misses". Not once, but several times, some times just a few hic-cups or on up to maybe 3-5 minutes. It then resumes running smoothly. It can occur during climb and cruise. I don't ever rememebr it occuring during descent/lower power settings. It most recently occured following water ops yesterday. When the miss occurs, the RPM gauge stays stable, giving no indication of loss of RPM/power. Yesterday the temp gauge was slightly in the yellow band. I observe the 5500 RPM limitation and we set cruise power at 5000 RPM.
I routinely buy the 91 octance from the same gas station, yesterday it was bought at another location.
It would be nice to get to the bottom of why this occurs and what I can do to prevent it.
Any ideas would be appreciated!
Reid
  • Re: 912 ULS Engine "missing"

    by » 12 years ago


    Hi Reid,

    Have you popped the carb bowls off yet to take a look inside? This would be the first thing I would do.

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


  • Re: 912 ULS Engine "missing"

    by » 12 years ago


    Hi Reid,

    Agree with Roger and look for water in the fuel system. Do you drain fuel before every flight and look for water?

    Jan

    Thank you said by: REID PETERSON

  • Re: 912 ULS Engine "missing"

    by » 12 years ago


    Thank you Roger and Jan. No, have not checked carb bowls. Unfortunately, I am not an A & P and if I did pop them, I would not have a clue what I was looking for. :( But, I do have a good Rotax mechanic that can do it. RE: water in fuel is a possibility. We do faithfully drain the sump. Airplane was not flown for about 3 months due to snow/cold up here in MSP and this recent event occured within the first 4-5 hours of operation this spring. Also happened right after some water ops, but I don't think that is a player in this problem because we normally do plenty of water splashes with no problem.
    I should mention also that this plane has a Sensenich 3 blade prop installed for better seaplane ops. The "missing" never occurs during take-off run, and most often happens sometime during climb and level cruise. But, like I said early, we can go hours and hours without a problem, with the engine running smooth as silk. Frustrating.
    The plugs were new and gapped correctly and all wires check only 15 hours ago. It would seem to me that it is not an electrical problem, but fuel. Never discount, though, a intermittment loose wire. Thanks, Reid

  • Re: 912 ULS Engine "missing"

    by » 12 years ago


    Interesting article from Van's; Start reading from page 9 - Growing pains.

    Van's had about the same problem with the 912. By pulling the primer they discovered that one carb was running lean. After opening the bowls they found a "flake".

    Thank you said by: REID PETERSON

  • Re: 912 ULS Engine "missing"

    by » 11 years ago


    Thanks for the replies. Popped and cleaned carb bowls. Some little flakes, no water. Pulled plugs and all were OK. Mechanic adjusted some carb and throttle linkages. took plane up for test flight and it presented same symtoms as before. Question: I have a Sensenich 3 blade prop. What take-off RPM would be expected? Seemed like we had less power after all the cleaning and adjustments.
    This coming week, plan to replace all spark plugs, even though they have only 15 hours on them. Ordered new carb floats and needles even though they were replaced 3 years ago or so. Plan to check more electrical stuff also.
    I would have bet that it was a fuel issue, now not so sure.
    Running out of ideas. Any more thoughts?
    Thanks, Reid

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