Sorry mate but sounds like things are getting beyond the realms of my knowledge with these motors for the time being.
Maybe posting some more info, mileage, age, history, mods from standard, I'm sure the more clued up members of this community will be able to shed some light on the subject.
The spark plug on mine is black on the outer edge, darkish grey tip, & white insulator.
Is your plug completely black all over?
I hope it's a simple fix for you like your air filter was. Fingers crossed.