Gspar,
On my forum, read tuning the Walbro carb. I gave made pictures of my spark plug readings, and cylinder head temperatures that went with the readings. Also I managed to get a good indication of the difference between a cold plug, and rich settings.
http://www.prme.nl/forum/viewtopic.php?t=10

I hope this helps in determining your problem, and get your engine tuned well.