The soot could be an indication that it is burning oil, however it could also be an indication that your mixture is a little too rich. That would be resolved with a simple caborator adjustment. I would further say, and I'm sure that there are some purists that would disagree with me, that your engine is 30 years old and if a little bit of oil is getting past the rings it wouldn't be the end of the world. Unless it looks like you are crop dusting back there and is a power loss I wouldn't worry about it.