Gas mileage woes

1991 Toyota Celica, 4 cyl. 2.2 lit, manual trans, 200,000 miles.(Not the original engine, however).

I just bought this car hoping to save gas and the engine runs like a top, as does the transmission. It's a fun car to drive too.

However, it is only getting 15 miles to the gallon and blowing a gasoline-smelling black soot out the exhaust pipe. The car sat for a while without being driven, but I also know the girl before me was putting fuel injector cleaner in it. I put one bottle in it this most recent fill-up.

Anyway, do you think I can replace these fuel injectors myself? I change my own oil, but that's about it typically. I just can't spend much money on this car.

Any tips for what else it might be or what might fix it appreciated.

- k