912 run rough on climb or heavy load

How old are the spark plugs, if over 100 hours replace as some may be intermittent under a load. Make sure all the plug wires are not touching the intake manifold.I’d check the balance the carbs at 5k rpm also. Keep an open mind and you’ll figure it out. Just my thoughts.

Dave, I will ask again, maybe I was not clear, how does your most recent annual differential cylinder compression test- by cylinder- compare to the previous test? And on a related note, could you have you recently inadvertently allowed air to enter the oil system?

Tangential thought - engine mounting rubbers???

Were they replaced at the last 5 year service interval?

😈

I chased a problem like this on a 912ULS for a while.

I found the issue by doing a more in depth fuel delivery test.  I took off the fuel bowl to the left carb put a bowl underneath ran the pump and timed the delivery and volume of fuel in that time, then repeated that for my right carb.  I found that I had less fuel flow to my right carb.  I took it off the engine and even though I had thoroughly blown out the fuel inlet before I really cranked on it with air and a small pice of what looked like wood came out - no idea how that got there.  Problem solved 

I saw a similar fuel issue on another plane with a 912ULS and this time there was a bunch of shredded plastic in their fuel line, that was a scary one...

I would definitely try the one by one carb fuel delivery test.

                 I know it has been a long time but I wanted to ensure the cause was accurate and to update this post with a proper resolution. So the resolution was actually a pitted exhaust valve. I know what you are thinking, lol. That should have shown up when I did the leakdown test before. I have no idea why it did not show while hot which is the recommended way to do the leakdown from what i understand. In any case I connected my leakdown tester while the engine was cold and noticed a drop (can't remember the exact values at the moment). I had removed my exhaust pipe to better access my lower plugs as I was again changing and gapping them and I could hear air. Obvious next step was to pull the head. which I did and the pictures are attached. I was unable to 'lap' the valve for obvious reasons so I replaced it and have not been able to reproduce the issue since. I want to thank everyone for the suggestions and guidance as all the information helped me better understand this engine. 

               One final note. since I acquired this engine (which was sold to me under the pretense that it was 'zero timed' from a certified rotax mechanic and instructor) it has always been stored in a hangar (no heat) and flown very regularly (approx 2-3 times a week).