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.

Hi Michael.

As I understand it:

The Float Bowl Vent Lines go from the nipple on the Carburettor - to the nipples on the Rotax Airbox, in a smooth curve - check our RO's video  above. You can clearly see how the Vent Lines have been installed.

The Airbox Drain Lines can be routed down to the lower engine bay/exit air stream. Pressure differences, between carburettor and lower engine bay/exit air flow will have little if any impact, as the Vent Lines will be equalising whatever pressure is in the Airbox with the Float Chamber.😈