I was going by the following site on reading the plug. Mine looks pretty much like the first example. "brown or light gray".
http://www.ngksparkplugs.com/tech_su...qs/faqread.asp
I'm trying another Carb tonight. Should eliminate this possiblity.
Also, I may have not mentioned that the backfiring is out of both exhaust and really doesn't take "hard" engine braking, just moderate.