4.6L plugs are black, runs rough with no acceleration on 1994 Mercury Grand Marquis

I changed the fuel Pump and got no good results.

2 answers
Have exhaust system and cat. converter checked for possible poor flow\partially plugged condition. This will cause backpressure and poor running. Is your check engine light on?
scan codes and post so we can adv. also ck fuel pressure,it may be to high due to bad fuel pressure regulator