These plugs were pulled after only 10 miles of driving, the majority of which was spent above idle, including some WOT blasts. Motor is a 383, Dart Iron Eagle heads, Holley 780 (69 pri/76 sec), and a 472 (69 Z) intake.

The plug on the left was in the #2 hole, and the plug on the right was in the #4 hole. Both look like the motor is running too rich, but it looks like one side of the carb is richer than the other side as #2 and #4 are fed from opposite sides of the carb. The other plugs also follow the pattern.

Idle exhaust has a strong fuel smell, but does not burn my eyes. I am thinking of tightening the idle mixture that feeds #2 by an 1/8th turn and going down to 67 jets on the primary. What do you think?

Plugs are NGK UR4. I also have tried AC R45TS with similar results. Would an NGK UR45 or UR5 be a better choice?

http://www.staff.uiuc.edu/~mwh/camaro/images/plugs.jpg

I would try another plug in that cyl and see if it happens again before you change any thing.