1991 Nissan Skyline sputtering

1991 Nissan Skyline GTS I-6 mileage: 58,000. My question is this. I have been having some trouble with my car sputtering very badly. It really acts like it is not burning all the gas. I changed the spark plugs and fuel filter. This did not help. Then I changed the 02 sensor. This did not help either. There is a really bad exhaust smell, like un-burnt fuel and there is some really black suit all over the back of the car. The car does not run hot, but the exhaust pipe is hotter than normal. One time recently the catalytic converter light came on. Could it be a problem with my ignition coil or something else? I am sure that the fuel is getting there ok, but I just don't think it is being fully burnt. Please help. Any help or advice you can give will be greatly appreciated. Thank you