912ULS Engine rough running in flight

Has any recent work been done on the engine or carburetors?

The carburetor vent lines are likely routed into the slipstream when they should be placed in a neutral pressure zone. Routing them into the slipstream creates a vacuum effect in the carburetor float bowls, which can draw fuel out and lead to fuel delivery issues. This problem only occurs during flight or takeoff and can’t be replicated on the ground. What you’re describing lines up exactly with that behavior.

It’s a common mistake for people to assume these vent tubes are drain lines, so they route them straight down the firewall.

Here is a video that provides more information related to the issue - https://www.rotax-owner.com/en/videos-topmenu/rotax-minute/454-carb-vent-lines

Thanks very much for your response. I'll be with the aircraft again early next week but our aircraft has a Rotax airbox and vent lines fitted. When I removed the L/H carb float bowl I did observe that there was a depressurization noise and a rush of fuel which didn't happen when I removed the R/H carb bowl. I'll update you once again when I'm at the aircraft next week. Really appreciate your help. 🙌

Following... Getting ready to install these vent lines.  I have a 912ULS with a Rotax airbox. I was not clear after watching the video how/where to route these lines when an airbox is installed.  I need to look at the Installation Manual.