So, despite the potential disgust of Brett , I did what I’ve never done before and tried head shims. With two, it’s now much happier and more civilised.
Changing the head clearance is a very useful adjustment, for fine-tuning. Unfortunately, for some strange reason, people do it by reflex. "I only got 13 appearance points, so I added .045 worth of head shims", that sort of reasoning. But if you are running 4-2 break engines for competition, then, knowing when to adjust it either way is pretty much mandatory. If you are already close to the right range, small changes (like +-.003-.005) are very powerful adjustments.
Unfortunately, it is generally not needed in almost any situation for sport fliers, only in the odd situation that Frank has. I have seen maybe 5-6 cases of misfires, etc, caused by excess compression, if your engine is not misfiring or quitting, it probably doesn't need more head shims.
And I emphasize again - FAI/ no-nitro fuel is almost unheard of in the USA, even 5% is rare and hard to find, either special order or make it yourself. 10-35% is extremely common and available anywhere. Almost all engines sold in the USA are intended to run on 10% or more and don't need more head shims to run correctly, and in the vast majority of cases, people would be *far better off* never taking the head off.
Yes, it comes with an Allen wrench, but that does not compel you to use it!
Brett