You threw me for a serious loop when talking about valve adjustments while running 2 cycle fuel. I had to Google your model and learned something new today. You say the plug was black... Any chance the rich condition caused the muffler to get partially blocked? I once picked up a chainsaw that idled and ran perfectly at partial throttle, then fell flat once I goosed it.
Well I think you learned me something too. I didn't put much thought into it until you commented. I've ran a br800 at my previous job and had no idea it had valves.