Valve Train Question, One won't hold adjustment-- Knowledge Base Chime In Options

[Robarabian]

A few weeks ago I posted a photo of some heads I pulled off a motor I had purchased from another member that had some heads with valves that were not in alignment with each other. They were GEX heads, and all the postings agreed, don't waste too much time on them. I saved them for another day.

I had a set of AA 2.0 heads (brand new) that were slated for another motor but this is a stop gap engine to use till that one is built. I swapped heads, cleaned everything, and re assembled. Valves adjusted and mounted to my test stand. Fresh oil, O rings etc...

Engine fired quickly and got the webers to the point where it idled reasonably and I was going to start adjusting them. At about the three - four minute point of idling and goosing the throttle a bit, I heard a bit of valve train noise, almost like a "POP" and I could hear clickcty Clack so I shut it down. I set the crank back to TDC #1 and checked the valves. (cold) 1,2,4 were good. #3 the intake valve was out of spec so I re-adjusted. re-checked the other three again, rotated the motor through and buttoned it back up. This was a week or two ago.

Today, I decided to tinker again and same thing happened. All was good, engine was running real smooth. Goosed the throttle, started tuning carbs and again, "POP". #3 is loose.

When I say "loose" it is going from .006 adjusted to .008 so it is loud enough to hear. Putting it another way, it sounds as a proper air cooled type 4 should, and then clickety clack, loud valvetrain. Cyl's #1, 2, 4 are in spec. I am setting Intakes at .006 and exhaust at .008.

I replaced all the valve adjuster tappets when I swapped the heads with stock ones, not swivels or anything weird.

This one is baffling me. Could I have valve adjusters out of alignment? Is the head just going through a normal cycle of stretching new valves when it got to operating temp? Am I losing my mind?

I appreciate the input from the knowledge base here. Im a garage mechanic, not a pro shop and Im only recently delving back into the mechanicals of the 2.0...