Rep Power: 425

Its been pointed out to me on 2 occasions now that the car looks like its running rich. It doesn't come out in my gas milage, but the inside of my exhaust is jet black and its been commented that it smells like it and sometimes blows a little black. Not like an Evo "so rich it blackens the bumper" rich, but too rich none the less. The engine, for all intents and purposes is bone stock.... so part of the question is whats the cause of this? And the rest of it would be how to fix it. Its not like I have a fuel system problem.... its like reverse of a problem. Can Timing do this? I wouldn't think so, but who knows. Plugs are Platinum +4s, they've got about 25K on them, I probably ought to pull them and have a look whats going on in the chambers. Any insight is good, but I don't have anything to tune it with, so I can't see it being a computer problem unless my ECU took a dump.