What is wrong with a 1991 Ford Explorer that uses a lot of fuel and smokes black but runs good and has lots of power?

Fuel mixture is too rich. Get this fixed asap as engine damage can occur. It can also damage the converter and O2 sensor.

Answer

Black smoke means you are burning oil. You might want to do a compression check on your engine to determine if you have a blown head gasket. Check you coolant and see if there is oil in it. If there is, then you defenitely have a leak. It could be a head gasket problem. Good luck..