I guess I came in late in this thread, I just saw the "mystery" part. Are you assembling after a top end rebuild? Are you sure you got your cam timing right? Did you turn it a full cycle by hand after assembling top end? Sounds like you gotta pull the head anyway to install the damper.... Look at piston and valves for contact, like Beezer said, possible bent valves. Post pics if in doubt, and we can tell you how to check your valve seating.
"Where ever you are, there you go..."