Crooked carb diffuser?

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.

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...

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

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

Was this related to your carburetor piston slide post about 7 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.  

Cheers