If you have dished pistons and a maxima head, ahem mn47;) you're around 9:1 maybe 9.5:1 compression, especially if the head was shaved during rebuild. Running that compression with dished pistons on crap gas is going to put the rings and valves through some wicked heat. I would suspect the valve discolloration is a result of this heat, as for the cam wiping??????? Maybe it was a result of a poor cleaning or rebuild, not sure. I hope a real engine guy can chime in and steer you better through this problem as I am a meer ammature.
Merry Christmas Hybridzers