Understanding Backfire

[SuperCheyenne72]

Long story short,

I just recently replaced my stock AC Delco sparkplugs with some "slightly used" (longer story) Splitfire plugs. In order to prep and clean my Splitfire plugs a bit, I used some WD40 and some Carb and Throttle Cleaner. I then gapped the Spitfire plugs to match exactly what the ACDelcos were at.

I pop all 8 in, and try to fire it up... it takes a while but doesnt catch. After pumping a few times it finally catches, but VERY rough... only to POP loudly and die. This goes on a few more times but never gets better.

Finally I decide to just replace the old ACDelco plugs back in.. and sure enough, it fires right up, with no problems.

So what caused the backfiring? Could it have been the slight moist residue from the Carb cleaner fluid still in the "spark zone" of the Splifires? Or could this have happened if one or two plug cables were not "snapped" in all the way?