Here is what has happened the last 4 times I've used the plane. It's a 914 ULS, 2016 aircraft. The engine has been running fine for a year (carb issues a year ago, carbs replaced and has worked without issue since then).

The plane starts normally, and then warms up past 50C. I then start the runup. The rpms increase into the 3000's, and it's here where the engine stumbles roughly. As I push the throttle further up to 4000, the engine runs fine, and even at full 115% power application there are no issues.

This stumbling appears to stop after running the engine further. I have flown 3-4 times since this occurred, and no issues have occurred on take off, in flight or landing.

I have uploaded video of the stumbling on runup.

https://www.dropbox.com/s/ugbiv7zzzf3x7ux/914.MOV?dl=0

Any ideas? It's seems strange that it only occurs in the 3's, and not up at full throttle. It is a very pronounced stumble, so I'm concerned it is something that could get worse. Thanks!
  • Re: Stumbling 914 only at runup

    by » 8 months ago


    It sounds like it is running lean in the midrange before the engine has fully warmed up.
    Until the Intake Manifolds reach operating temperature some of the fuel sprayed from the carb jets remain liquid and wet the inside of the manifold instead of vaporizing.
    Once the engine heats up another 20 degrees the issue resolves itself.

    Did this start after the weather turned cold?
    If so, you might want to set a richer mixture for the winter.
    The cold dense air can burn more fuel than thinner summer air.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: Greg Stowell

  • Re: Stumbling 914 only at runup

    by » 8 months ago


    Thank you Bill. This did start about a month ago, when the weather turned cold. It's odd that it didn't happen last winter.

    When you say set a richer mixture, are you talking adjusting the idle mixture screw on each carburetor ?

  • Re: Stumbling 914 only at runup

    by » 8 months ago


    Try setting both needles in the #3 clip position.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

    Thank you said by: Greg Stowell, Bill Hertzel

  • Re: Stumbling 914 only at runup

    by » 8 months ago


    Thanks Roger. So another dumb question. What's the purpose of the idle mixture screw vs a pin setting? Is that to make a minor adjustments between pin settings?

  • Re: Stumbling 914 only at runup

    by » 8 months ago


    The idle mixture (percentages of air and fuel) are controlled by fixed idle jet, idle bypass, idle restrictor, and the idle air-bleed. The amount of mixture is controlled by the idle mixture screw.
    The main jet needle controls the fuel flowing through the carb at higher rpms than idle.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

    Thank you said by: Greg Stowell

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