I would think your Enya would turn that APC 12x6 no problem in the 10000 RPM range. Are you using a tach. for tuning? Too large of a dia. prop. will produce a knock. Personally, I have never been to keen on Enya plugs. I would first try a OS F plug, check the valve train and take it from there, then try a smaller prop.

One of my OS 120's developed a knock last year that was different from an ignition knock. Turned out to be a bad rear main and one cam bearing was iffy. I concluded it was most likely a bearing problem because the RPM had dropped considerable and it seemed to be running a little warmer.