Hello Rotax forum members,

I have been battling a buzz that starts at 5000 RPM on my airplane. It's not really that bad and the engine is very smooth below 5000 RPM but it bugs me (I have a couple of analog gages that rattle a bit). BTW, this is my first 912ULS so maybe it is me not used to it but I have gone up with other planes and they seem better (although that could be airplane related)

I have gone through things like gearbox clutch check, carb overhaul and ultra picky balancing, prop dynamic balancing, ignition system checks, etc, etc.

I have been doing some vibration spectral analysis and found that the buzz I feel is right at engine RPM. The airplane feels fairly good up to 5000 RPM because a little over that it seems there is a resonance in the airframe. But I won't go much into detail and stick to the original engine related question.

The engine RPM vibration spike is not abnormal (per RPX, maker of the Dynavibe) but should go away at high cruise RPM and mine doesn't seem to. My carbs are perfectly balanced but during overhaul I found that the pressed in diffuser in one of the carbs is slightly rotated. Can this be an issue in causing flow imbalances? How can this diffuser be rotated and is it something I should bother to try to correct? Picture attached.

Best Regards and Thanks,


8998_1_Rotated carb diffuser.jpg (You do not have access to download this file.)
  • Re: Crooked carb diffuser?

    by » 12 months ago

    That is an interesting find!


    A CSIR should be created so that Rotax is aware of the issue you found - https://www.rotax-owner.com/en/support-topmenu/csir-service-information-report

    It is very likely the offset diffuser is the cause of the vibration you are experiencing. I would imagine that carburetor is running slightly lean compared to the other carburetor as the fuel flow from the float bowl would be slightly interrupted (especially at that RPM range). We have seen similar issues when the piston slide is not installed correctly and is twisted. Unlike the 912UL (80 Hp) or the 914 (115hp), the 100hp 912ULS diffuser is pressed into the body of the carb, and this would have had to occur when the carburetor was initially put together.

  • Re: Crooked carb diffuser?

    by » 12 months ago

    I have a crooked diffuser in one of my carbs too...  since they are now pressed in at the factory, there's not much to be done about it, I was told...

    I have not noticed the sort of issues you describe, but I am a bit mystified why they decided to go to the press-in design if errors in the construction cannot ever be corrected.

    Also, be aware that you will never get a response to a CSIR, should you choose to submit one.  Ask me how I know...

  • Re: Crooked carb diffuser?

    by » 12 months ago

    The carb can be sent into LEAF (Leading Edge Airfoil) and they will press out the diffuser and straighten it for you https://www.leadingedgeairfoils.com/ 

    However, this would not be under warranty and you would have to pay for the service. Give Brett a call and he can provide you with a price - 1-800-532-3462

  • Re: Crooked carb diffuser?

    by » 12 months ago

    Tyler Hathaway, Who did you submit a CSIR through? - RFSC?

    Was this related to your carburetor piston slide post about 7 months ago??


  • Re: Crooked carb diffuser?

    by » 12 months ago

    Hi Christian

    In my experience it is not the carb it is most likely related to a gearbox wear issue.  My question is did this ever work right or is this new?  if this is something that developed then we can discount the carb and look at what has changed in the gearbox.  Most often it is a result of wear between the dog gear and the main gear and can be related to friction torque settings in the gearbox itself.  I assume you have a ULS that has an overload clutch in it also.  

    There is always more "buzz" in the 100 HP engine due to the high compression pistons.  If this is an older installation consider the age of the rubber dampers between the engine and the airframe.  If they are too hard from age it can affect the feeling of airframe to engine.  As a general rule those should be changed every 5 years regardless to allow for aging.  



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