There are two in my club in identical Cermark Sukhois, the original one. Both with a Pitts style muffler, engine mounted sideways. Both run great on 10% fuel. The other guy tried 15%, had all kinds of problems. Switched to 10% and had much better luck. Mine's always been run on 10%. It took a while with tiny adjustments, but now both engines idle, transition and run great.

Excellent engines, perfect for these aerobats. We run 14X6 props, usually. Lately he's been hovering with a different plane, and is talking about using a 15X4. We tried the recommended 13 X 8, makes it difficult to land, but it spools up instantly. The 15X6 I tried was slow spooling up, but provided excellent braking on approach. The 14 X 6 is a compromise, good acceleration, and a little braking on approach.

Try a different muffler, or different fuel, or prop or something.