That is a hot plug, stock was a J11, which they don't make anymore, the J12 is the replacement. I run 9s with a Holley double pumper and a big cam that is fat on the transition and on the rich side at part throttle and my plugs are always clean. I'd suspect you are WAY rich (burn your eyes rich) or you have an ignition problem.

I would NOT put an 18 plug in there, I wouldn't even run the 14s TBH. What are you running for ignition system? What is your timing set at?

I had an engine once that would carbon up the plugs like that but ran great. The stock distributor had about 10 thou of play in it, opening and closing the point gap as it wobbled around. Never back fired and ran clean and mean though. Put a new dizzy in it, ran the same but the plugs stayed clean looking.