Hello all,

I've had an issue with my 912ULS serial #5652370 for quite some time now. I'm about to give up and just keep flying it.

Engine has 1,190 hours on it and has had a noticeable miss from 2100 to 2500rpm.

Top end has been de-carboned and the intake valve seals have been replaced.

Engine ignition check is only an 80rpm drop on each ignition. There was a noticeable miss On the "B" ignition until I set the trigger gaps to the recommended .012" gap per the heavy maintenance manual. The ignition with the miss trigger coils were at .008".

Since the miss began and prior to it both carbs have been rebuilt, floats have been checked and carbs have been balanced.

Spark plugs, wires and caps have been replaced.

The only thing that hasn't been done is to replace the CDI boxes, but I don't want to do this unless I can prove they are bad. Other than the low rpm miss the engine runs flawlessly.

All maintenance items mentioned above were done due to time and hours.

Any ideas out there? I'm out of them.


  • Re: Engine Miss 2100-2500rpm

    by » 5 months ago

    Here are some thoughts:

    When you get the misfire, is it still worse on one ignition or is it similar on both? If you immediately stop the engine when it is misfiring, what does the fuel level look like in each float bowl? Are the small drillings in the venturi near the butterfly clear and unblocked?

  • Re: Engine Miss 2100-2500rpm

    by » 5 months ago

    Is it an actual engine miss or a vibration? The reason I ask is to help rule out the gearbox. With 1190 hours when was it overhauled?

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

  • Re: Engine Miss 2100-2500rpm

    by » 5 months ago


    It's the same on either ignition. I've checked all the ports and passageways multiple times.

    Same thing with the floats! They are good as well as the level. Having had 20 floats sink on me over the years, I've gotten good at this task.

    Thanks Kevin!

  • Re: Engine Miss 2100-2500rpm

    by » 5 months ago


    It's an actual engine miss. You can hear it and see the engine shaking.

    The gearbox was rebuilt at 650 hours by Ronnie Smith at South Mississippi Light Aircraft.

    Thanks Roger!

  • Re: Engine Miss 2100-2500rpm

    by » 5 months ago

    Having had a number of faulty ignition modules over the years I've disassembled some to look at the circuitry and I haven't seen an issue with them that would cause a misfire over a particular rev band.

    I did have a failed inlet valve where the sealing face was worn/deformed. That started off as a misfire that rapidly got worse to the point where I was scared to fly. It was only diagnosed when the engine was stripped down and rebuilt but that was when my oil consumption problems began. In the end I just purchased another second-hand engine as it was all taking too long to diagnose and repair.

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