Quick update re this afternoon's shenanigans.
With the valve clearances loosened out to ~0.1mm and tweaking the mixtures in the starting gate with the tacho to get a baseline setting, I just had 3 x 20 minute flights with hardly a beat skipped.
Considering the new ring has about 10 minutes and the mixtures were "about right" I'm not complaining at how it ran.
There still may be an issue that requires the valve clearance to be set so loose, and I may tighten them back up to about 0.06mm if I have some more sucessful flights but usually I set the valve clearances ~0.025mm. The is some small wear under the rocker and maybe the rocker is shifting causing the valve to ride or maybe the lifter isn't always going all the way back down if there's a lip where it rides??
Still more speculation but fingers crossed it seems to be running fine.
What valve clearances are others running???