Getting bad gas mileage and very little power out of my 1990 Ranger on 1990 Ford Ranger

I recently redid my entire engine so it should be running good as new. However, I have been getting around 12 mpg and it's COMPLETELY powerless. Pretty much the only thing I didn't replace when redoing the engine was the fuel injectors. Any advice on what I could do?
Thanks, Sam

Asked by for the 1990 Ford Ranger
What condition was it running like before the rebuild? Any chance the old engine was dumping fuel/oil into the exhaust causing converter problems(clogging & restricting flow on rebuilt engine) IF that's suspect, try 'creating' an exhaust leak before the converter (let it breath) and test drive to see if that helps , if so repair/replace neccessary parts.