i know that this is 3rd gen and ive had a few but i cant find any other forum as reliable as this one and well......... its still a chevy

okay i have a 1989 chevy 2500 with a 350tbi 200,000 miles i just did a basic tune up oil, oil filter, air filter, sparkplugs and top off of other fluids. my problem is that it smokes (about as much as a regular car in the cold) and seems rich, all the old spark plugs where black and smelled of fuel. it runs great and has plenty of power but its seems rich. what should i do?? im planing to have a valve job done soon. and in the next couple months replace the o2 sensor then the injectors.

is there a way to adjust the air/fuel myself with a tbi?
or is it something else?

i also have 1 other question my exhaust goes 2 into 1 and then the o2 sensor. can i do a true dual exhaust?

thanx for the help also feel free to guide me in the right direction of other post that address my problems i looked but didnt find anything.