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.