Magnus,

If you haven't tried it yet, consider using the Nelson FAI head with about 0.020" to 0.024" head spacing. (Sorry, I don't know the metric conversion offhand.) Also, the flat-wound GloBee plugs seem to work better with FAI fuel. I don't know if either of these features would help as much at 19,000 rpm as they do at 27,000, but it's worth a try.

Gee, I feel like I'm corrupting the young here ... didn't Socrates get in trouble for that?

Mr. Cough Syrup