1998 Nissan Sentra Rough start, misfires

I had a misfire in cylinder 2 while driving on the highway about a week ago, noticed considerable decline in power and car started riding rough. Replaced spark plugs and cap and rotor, wires were checked and ok. Still having misfires, only in cylinder two. Still riding very rough, starting very rough. A couple times had trouble starting it at all. Checked new spark plug in cylinder 2 and it was covered in dry carbon. Car burns oil, always has since 20k. Where do I check next? Could it be the fuel injector? Does the entire system need a carbon cleaning? Thanks!