Oh dear .... all this on a 1940 technology , low compression , glorified VW industrial engine !
Andrewr I wouldn't bother ... these guys have been cult members pushing this barrow for awhile .... clearly they know best and pilots have been destroying engines prematurely for 70 years...
Airag don't discourage Andrew from learning. He stated earlier that he wanted to do that.
All 4 stroke engines are very similar in that they all suck,compress, bang and blow. Inside the engine there are many variables such as stroke, valve position number and size, piston top shape and number of rings. Also there are many different internal head shapes. This lists some of the variations.
Outside the cylinder it becomes even more interesting. I guarantee the fuel and air induction system in your beloved VW is nothing like the IO520. the same applies to the ignition system.
You could be the worlds greatest expert on VW engines but that would not help you sort out why factory new IO520s run rough at lean mixture settings. The reason is quite simple and logical for those who want to learn but you must understand how that particular engine, and many like it, recieve the air and fuel.