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Hello guys!

I am having some issues with my 912ULS. The engine is 1400 hours with no issues up to now. Some weeks ago some vibrations below 4500rpm appeared... I did a carb overhaul and synchro and the engine is running fine at low and high rpm however when I pull the throttle back from cruise rpms to 4200-4300 the engine starts to run rough with some noticiable, not very high, vibrations. If I push the throttle a bit these vibrations are highly reduced and the engine runs smooth... Also if I apply throttle from idle to 4300rpm the engine is working fine... It seems that the throttle could be the cause of it as I am only having this problem when throttle back...

Do you have any clue about potential causes??

Thanks a lot!!

  • Re: 912ULS rough at throttle back

    by » 5 weeks ago


    Many owners report a vibration at the rpms you have listed. It's the nature of the beast. Harmonis and even the prop and wind can play a part. Some engines in many types of applications have a harmoic vibration at rpms that a mfg may not recommened running at. Most engines are designed to run at certain rpm's for their specific application. This is one reson we don't cruise at those rpm's all the time. Fine for coming into a patteren, but since you know where you vibation is then it's easy to avoid.


    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: RotaxOwner Admin

  • Re: 912ULS rough at throttle back

    by » 5 weeks ago


    Thanks a lot Roger! It's good to know that it is not a severe problem... I was wondering why I am only finding this rough running when I reduce power and no when adding power... 

    As you are commenting I am avoiding running the engine at those problematic rpms, using 4800 for cruise and below 4000 for descents.


  • Re: 912ULS rough at throttle back

    by » 5 weeks ago


    It was running smooth for 1,400 hours at all throttle settings, then began running rough and vibrating? I don’t understand how that could be a normal characteristic of an engine in an unchanged configuration.  Why would it not do this from day one then?  Something has changed.  


  • Re: 912ULS rough at throttle back

    by » 5 weeks ago


    Thanks a lot Jim for your answer!

    I am trying to find the possible cause, that I thought was on the carbs, but after the carb overhaul I improved the issue but not removed it for this 4200-4300 range when throttle back. So I need some advice about what my next check should be....


  • Re: 912ULS rough at throttle back

    by » 5 weeks ago


    The issue is anyhow related to carburetors, as the performance was improved after carburetor's overhaul. I have smilar type of issue with unstable RPM at around 4000 RPM settings. Carburetor overhauled was not worked. In last I replaced the carburetors and the problem got settled. If possible go with carburetor replacement or change the type of fuel for few operations. I mean from petrol to AVGAS or so. 


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