Gas Burning Problem... HELP!

Hey guys. I've been searching around for a while now, and I came upon this forum. I really need help with my 240... if you guys wouldn't mind. It's a 1990, and the main problem is that it is getting horrible gas mileage. Another problem it is putting out a lot of black smoke... which I know isn't normal. Me and my brother have changed the plugs and plug wires, the 2 air filters, oil filter, oil change. I just can't think of anything else to do. I would really appreciate any input any of you have on what I should do. I've also tried the fuel injector cleaner, that you put in with the gas... that helped a little, but only lasted a few days. Please post ASAP with any helpful hints... thanks guys.

Mike

maybe the fuel filter, cat converter, and check if they car is overheating.

Time to do a tune-up. Search on here and I beleive there is a few threads with all the info.