I'm sorry, but it's still not exactly clear what you're referring to in the picture. I see some pitting between the valves directly across from the spark plug that looks like the result of detonation or corrosion (hard to tell) and I see carbon deposits (pitch black) around the perimeter of the exhaust valve and near the spark plug. The former is in a very unusual place for detonation damage and the latter is because you're running too rich. Is there much carbon on the piston? What compression pistons are your running?