TW,
Something is causing the valves to get extremely hot. It could be anything in the combustion process. I have also seen it in marine engines when water gets into the cylinders through the exhaust but I doubt this would be the case here. Did you change anything during your "freshen up" that could keep the valves from dissipating heat ? Like a much narrower valve seat than you had before ? Anything else change in your combination ? Not enough timing can be just as bad as too much. Vacuum leak ?
Just thinking out loud
