1991 Pontiac Firebird idles rough

Engine Mechanical problem
1991 Pontiac Firebird 6 cyl 130000 miles

This problem has been occurring as of the last month, my gas mpg is down to 10 when it should be around 16-20. When idling it will sputter, (i have a cat back exhaust) black smoke will come out, a flame will come out of the cat if I don't rev the engine up, if I rev the engine a little it will run better for a few seconds. Ive replaced o2 sensor and the cap and rotor. How ever it still idles really rough. If I dont do anything my cal will die out. This problem has started with no really warnings or signs. Ive also replaced the tps recently, and replaced air filter. Any help would be really appreciated