So I've noticed this for awhile but haven't done anything about it.. If I have my car idling on my driveway for a few minutes, this will start forming under the tail pipes.. Mostly on the passenger side one.
I have a Magnaflow exhaust with a stock header/mid pipe.
Running a little rich there. Could be build up in old tail pipe sections getting blown out???? Otherwise, theres too much gas being dumped in one or more of the passenger side. have you ever had the O2's replaced?
I'm going to rule out oil. Thats too black and sooty looking.
Ya my friends El Camino did that all the time till he changed his carb and tuned it right. He'd shoot black crap all over the car behind him. SO I'm gonna agree with MustangLX-5.0 that you're running too rich