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Yesterday we got ready for a flight and right after starting the engine it ran very rough. Switching ignition made no change. A look at the egt's showed no activity at cylinder 3 and 4 (right side). So only left bank was doing something. To find out what could be wrong we pulled the bottom plugs at took al look with a handy camera connected to my phone. Nothing strange showed up. Removed some baffling and pulled the top plugs. Again nothing. So took off the inlet manifold. This showed up.

https://imgur.com/zNXKpl3
Not very clear view but something seriously wrong in cylinder 3. So pulled the head.

https://imgur.com/9VCDsFd
Valve seat is hanging loose around the valve stem.
Last flight a week ago and nothing wrong. We did 3 takeoffs with full stops in between and returned back to homebase. At arrival engine ran normal and back at the ramp ran normal at idle and both ignitions worked as they should.
Engine has about 70 hours and the heads (EMPI) 60. They replaced the Mofoco heads after 10 hours. Last maintenance 20 hours ago with oil change and valve adjustment to manual specs. Previously running with a little more, because we noticed tightening up after a few hours. We thought that after 50 hours it was a good moment to put it according specs. Measuring now showed tight exhaust valves on 3 and 4 and almost tight on 1. My guess, according data analysis, is that the tight valves caused the CHT's to rise 20 degrees C and probably even more at the valves seats resulting in overheating that spot.
I think the play for the exhaust according the manual is too tight or check it every 10 hours instead of 25.
We're battling high CHT's from the beginning. Made some mods with moderate results. About 5 hours ago we changed to the 2.5 needle. We now get 3300 rpm in takeoff with better performance. CHT's in climb still goes high, but it takes longer too reach.
We have one good Mofoco head left and after checking the valves put in on and hopefully get it flying again in a few days.

Wish I had good advice. Thanks for sharing the pictures. I believe you're in Holland? Do you get 100LL over there? My cylinder heads are not black at all inside--more of a dull gray that I assume is from the lead. Is that bottom spark plug damaged? What about the pitting below it? Is that a result of the failure? I was initially suspicious you experienced detonation, but I guess some metal must have come off somewhere during the failure and bounced around causing the "pitting" and other damage?

My experience is also that the valves clearances tend to decrease with time. I interpret this to mean either the seats or valve faces are wearing.

I try to keep my CHTs under 400F (~205C) but occasionally go over. When you say you've been battling CHT's, are you spending a lot of time hotter than this?

I have Empi heads and the exhaust valve seats were warped after 30 hours. Had to regrind the seats. Can I ask you what happened to your Mofoco head?

We have 100LL here in the Netherlands, but we're running 98 auto fuel. I guess the black is from running rich and bad combustion during the short time the engine ran. Sparkplugs aren't damaged. The 'pitting' is some oil what shines up in the sun.

During climb out temps of 1 and 3 go way over 400F. 3 is goes to 450. We're lucky to get below 400F in cruise at MTOW.

It's my experience too that valve clearances decrease with time. Especially while still 'running in' the engine witch takes up more than 25 hrs.

I have to check the valve seating on the remaining 3 'good' valves in the EMPI head.
Th Mofoco head was ruined in the first 5 to 10 hours due to a too lean mixture and too advanced secondary ignition. Bad combination.

Also due to leaking exhaust valve that gave very high temps in that area I think.

What is your opinion, quality wise, between the Empi and Mofoco heads? I'm not too happy with the Empi.

Empi looks better and I think the material is harder. The spark plug threads in the Mofoco where gone after a few times unscrewing.
But we ruined both, so it doesn't matter much.