Thanks Bax, for the very helpful and complete answer.

I'm actually unsure how much break-in time this engine has had. I'll start treating it as though it's new, as I should have from the beginning, and run it on the bench for a while before taking it back to the field.

OS suggests that this engine needs at least 18% oil in the fuel. I'm using Cool Power 4-stroke 15% nitro, which has 17% oil if my online reading is correct. Is this adequate for the engine? Sometimes 1% is insignificant, and sometimes it spells the difference between success and disaster.

I presume 9000rpm is normal for this engine on the bench with a 15x8 blade.