Rotax problem

Hi everyone - here’s hoping I can pick a few brains...

We have a Rotax 912ULS powered aircraft that’s been having a few problems lately. We run the aircraft on E10 petrol, but it’s been cold lately and don’t think the problems are vapour lock - though we’ve experienced that in summer.

The most recent experience of this problem was as follows:

The aircraft had already flown three separate one hour sectors that day. On the fourth, at 300ft after take off with wide open throttle the engine surges from full power to no power, full power to no power repeatedly. It does this quickly enough that the engine doesn’t have chance to come to a stop between surges of power. We’ve checked all hoses are tight. Fuel flows from the tanks freely. After I brought the throttle back from wide open it ran smoothly in the mid ranges for the short period of time that it took to get it on the ground.

The fibreglass wing tanks have been resealed recently.

The OAT remained around 10 degrees Celsius for the day.

I think we can rule out ignition, so it’s down to air and fuel. I have a feeling it’s a problem with carbs. Having said that, we can’t 100% rule out vapour lock either.

Any thoughts gratefully received.

Disclaimer: not a Rotax guy.

Based on what you’ve stated (recent reseals), the first thing I’d check is filters, screens, etc. A clogged filter cycling to bypass (if it has one) and back that reveals itself under the highest flows points to a such. Second would be carb float but less likely based on what you’ve stated. I wouldn't fly until I was comfortable that the issue has been resolved.

Keep us informed.

Fiberglass tanks and E10!!!

Check venting is adequate. I had similar situation, every left turn resulted with restricted airflow and power loss. To turn left, I made a lot of right turns.