need help with carb/plugs tonight if possible

so i take my backup drag car/street car to the drags tonight and its running like a pile. it has a 351c with 4v heads, procomp intake, 650 hp carb, pretty much stock, normally runs 12.20s, was all over tonight. 020s/.030s/.080s, even 13.40s, change plugs and its right back to the 12.30s for 2-3 passes, then it gets worse n worse, when changing the plugs i notice every time the number 3 and 4 are like new and the other 6 are black/black and failled out. would that be the intake or carb? any help would be great, since i have my points race in the morning. this car is normally dead on with times. thanks in advance.