from talking to other LAMEs who have had a good record with Jabirus, it could possibly be the construction technique that is the root cause, and also, not having any idea whats going on with regards to mixtures and combustion events.
the crank cased are bolted together, no gaskets, just metal to metal and a thin layer of silicon between the mating surfaces.. something i have been told is a big no no in engine construction... leads to fretting etc...
Unless you are running a stupidly lean mixture and cooking your heads
Don't you mean, Not lean Enough?