New engine running rich
 

My 3rd gen has had a little bit of downtime due to college and work i just haven't had the money to fix my car. My 5th gear syncro was bad and after the car was sitting for about 2 years with me tinkering with it every once and a while i saw a deal that didn't look to be too bad. It was a j-spec engine with the tranny turbo's and everything from the michigan motor exchange. It was my first time pulling and dropping a rotary but it went really smooth. On start up i had a few little problems, one was my air pump siezed causing the aip pump clutch to start smoking(that had me worried a little bit). I decided to un-plug the air pump and just cap the end of it because i am running a straight down and midpipe now. also the bearing in the alternator is shot causing it to squeel pretty loud. i just ended taking the alt off the other engine and that problem was solved. I also had one or two more little problems that only had me stumped for a few minutes. but

This is my big one, The car is running rich. There is some black smoke but i know part of that is because of the car now being straight piped. The car smells strong and the biggest reason i believe this is beucase i have driven the car 120 miles while testing all my little problems out and going through an entire tank in 120 miles 120 miles/20 gallons.....6 miles a gallon and i've been taking it extremely easy on the car so those numbers jsut don't add up. What would be the main causes of this? I was thinking o2 sensor but that seams a little extreme to just be the o2 sensor. I called the local auto parts store and they said for an oem o2 sensor it would be $128 so i'm kinda holding off right now. The other thing i could only think about was the fuel pressure reg. but that's another $170. so i figured that i was ask just incase people don't think that this is my problem at all.

Any help at all would be great
Thanks,
Patrick