[Rich Biebel]

Sean they do tend to get looser rather than tighter.
BUT if the exhaust does get tighter, the lash is so small it can lead to early head failure as I have described earlier.

This is a fairly common issue on small engines with solid lifters.

It was new to me when I first started working on this equipment.

They run fine cold and after a warm up and maybe some load gets applied they start to run badly.....shut it off....cools down.....cycle starts all over again....

Onan flathead engines did this pretty regularly......They had a very wide exhaust lash spec at .013"

Exhaust valve needs to touch the seat every cycle to shed some heat of combustion from the valve....No touch.....valve overheats and it magnify's the problem.....

Small engines were a whole new world to me going back 15-20 years ago...

Chasing some of these problems has always been a challenge to say the least....