Here is a thread with information you may find applies to your situation... especially the burned valves. Pay particular attention to Maxwell Power's posts: http://forums.nasioc.com/forums/show....php?t=2401576
Start with this post:http://forums.nasioc.com/forums/show...2&postcount=82
and follow the subsequent posts paying particular attention to his mention of aging MAF sensors. I, for one, never considered the MAF Sensor as a maintenance item, but I do now.
While the entire thread contains more value than most, now that you have the engine apart there is a lot more in the thread you may find applicable besides the MAF Sensor info.