93civdx

Ok, i'm new here and i have been reading on this site for about 4 months now and i have really learned alot. But i do have one question. I have a 93 civic dx 4 door and i have an AEM CAI, dc 4-1, and a muffler. on my back bumper it looks like there is a lot of carbon build up. is it because my cat is clogged or i am just running rich. if so what do i need to do about it? any help will be good help. thanks ahead of time.