I find your statements not only confusing but disturbing. If the adjuster was all the way in, one would only have to loosen it to get clearance. What clearance are you adjusting them to and how often do you check them. Are you positive that you are adjusting the cylinder that is tdc and on the compression stroke? If you rock the prop a tad when you're at tdc and there's valve movement, you're on the wrong cylinder.I just checked the valves on my 1700cc vw engine. This is a full vw engine not the 1/2vw. But I found on each side one intake valve that the adjuster was turned all the way in. I needed a little more clearance and could not get it
I know next to nothing about these split case air cooled engines and have no problem admitting that. Why I post here. Why I refuse to try and built one to put into the air. I can do some maintenance but that is it. It would be nice if someone offered training for these VW engines that go into the air. I would be standing in line for that training.
I would run a compression check before I went anywhere (even around the patch). If it's had tight valves and bipasssing gas through them for any length of time, you have the potential for them burning,stretching, and/or breaking. A leak down compression checker would be best. Someone on the field should have one.
I hope you have a cht and egt monitoring system. If you get that baby to hot, it will fail.