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I've replaced spark plugs, rebuilt carbs with master kit,replaced plug cap ends.
Balanced carbs..Plugs were toast and the carbs needed attention so well spent.
Here is the original problem. Start out engine runs smooth at 1800-2300 applied power from 2400 to 3400 runs a bit rough but smooths out to 5400. Fly along for approx. 45 to 60 minutes at 5000 and reduce power to 4800 the motor vibrates and feels real concerning. To smooth out I reduce power to 4000 for a brief moment and back to 5200 and will smooth out again for a period of time. This was the original concern before the work I performed and still continues. This is a real gremlin. Is there a rotax guru in Oregon that I could contact for further diagnostics?
  • Re: Engine Strange Miss

    by » 10 years ago


    I what model of engine two stroke or four stroke .

  • Re: Engine Strange Miss

    by » 10 years ago


    If it is a 912 you will just have to go throw a hole list of things like any maintance person would if a two stroke most of this list would be the same
    Make sure air filter clean (changes mixture )
    Check Carby flow levels and flow needles for leakage being old and hard
    Check to see you aren,t getting carby icing not sure where you are flying
    Check fuel tank vent ?
    Check carby sockets for internal cracks
    Do a cylinder compression test hot and cold
    Check prop balance and torque prop mounting bolts ( a good idea to get prop dynamically balance on plane )
    Check engine mount rubbers that they aren,t bottoming out or old and hard etc
    Depending on the prop weight used on 912 the gearbox may need stripping every 500 hours to set up preload washer springs on dog gears it is very important and is most often never done this helps smooth out engine pulses . If has a slipper clutch fitting it must have torque checked . (If two stroke gearbox should be checked at least 250 hours ) this is best done by some one that has done a few be for so done rights.
    If your engine has had a prop strike at some stage of its life the crank will need a phase check if out of phase engine could run rough as timing and engine balance out and crank will fail as I have seen .

    That just a few things to check for a start

    Best to always sort the problems out on the ground first by strapping aircraft it up to a car etc
    If you feel it has a problem don,t keep on flying it get it looked at .

    Good luck
    dan

  • Re: Engine Strange Miss

    by » 10 years ago


    This may also be a prop problem. Get it dynamically balanced.
    Could be rubber engine mounts.
    Could be the gearbox needs attention, i.e. Shimming
    Carbs may need rebuilding

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


  • Re: Engine Strange Miss

    by » 10 years ago


    The motor is a 912S with 333hrs total. 3 Blade Warp drive.
    I have replaced carb sockets / Rebuilt Carbs with Master Kits
    Replaced Spark Plugs & Sockets / It does have the Aluminum Air Box All Connections Secure
    Balanced Carbs....It runs smoother initially than before but after that 45-60 minute run is when
    it develops that strange vibration. Can anyone point me to a Rotax Tech in Oregon I can take it to. A few of the A/P I've spoke with are happy to look at it but not much knowledge on Rotax.
    The Prop has been balanced according to log book appears prior owner must have dealt with the issue as well. I'm thinking gear box next...??? I appreciate any and all suggestions....

  • Re: Engine Strange Miss

    by » 10 years ago


    Back to the basics I'll post future findings. Thanks to all responses.
    Mike c.

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