Rich Fuel Mixture ¿help?

Sup yall... I have a lil 91 crx dx with the stock motor and all, and im not getting the swapp until this summer lol.. so for now is the stock motor... Im running a rich fuel mixture kuz of the black soot on my muffler, which Sensor is it? my mass air from sensor or o2 sensor?? or would i have 2 have it looked at 2 find out? thanx for any help! :feedback:

check if oyu ecu is throwing codes, if it is the o2 it will be throwing code 1