Carbon on Exhaust Valve
Whenever a four stroke engine starts to run ratty it is carbon on the exhaust valve. I am now running a YS 80 on RotorRage 30% heli fuel. It started running ratty at WOT. I took it apart and it was pristine new inside. No deposits on the head or top of piston. The ring looked new - still black. I thought I had wasted my time but decided to take out the exhaust valve since I was there. It was coated with carbon on the seating surface. This is a pattern for me anyway. It's always carbon on the exhaust valve.