OK; This is getting interesting. I would have thought you would have noticed a bad valve when you had the head off for the new piston work. I am guessing you did not check the compression before you did the work. That would tell you if it was bad before you tore it down. If not, I doubt it was a valve unless it got some foreign material under it when you had it down. Thank you for letting us in on this one, and please keep us informed. I love these mysteries.

 

It could also be a bent valve if you adjusted them wrong or didn't let them bleed down all the way before turning the engine over.
John

 

Just a last note to all. Thanks for the help. I pulled both heads, had a valve job done, new SE quick install adjustable push rods. She runs great and strong!!! I did have a burnt valve, and a leaking valve seat... Both were in the back head though.