You answered the problem in your first post. In many installations the idle needle CANNOT be set for best idle without going lean thru transition. Set the IDLE a bit more on the rich side and you should see a huge improvement. The problem gets worse as the nitro percentage goes up.

The plugs in this engine are not real fussy . I have had my best results with the Enya #3. It works at least as well as the OS F but lasts years longer .