RE: When does a four stroke need a new ring.
Cam timing usually just affects the top end or the low end, and does't change after it warms up. That engine's RPMrange is 1800 to 11,000, so as long as you are close to those numbers, the cam timing is OK.
Still sounds like a fuel issue, mix not the actual fuel. Marcel, is there any surging at mid range, in the air?
Very nice looking plane I might add
Don