If ralph worked on this engine all of the above is spot on already. Maybe some redline oil at proper mix and correct needle settings would cure it.
ORIGINAL: cuwaert
This is a reply to the thread.
I don't know anything about the American oil brands but I think you can eleminate a few possibilities mentioned here.
Check pre-ignition angle.
If you tune the engine so that it got it's maximum power and a steady idle then it's mixture is ok.
Now ,after some runtime, if the plug is still not ''coffee with cream'' it's eighter overcooled or the plug is too cold a type or both.
Check Zenoah specifications for what oil brand you should use and ratio.
Port timing seems a long way from home to me unless Zenoah messed up an engine specially for you.
But ofcourse , and you won't believe it, nothing is impossible, there are only more and less probable things.