93 CX, B16a, bolt-ons, chipped ECU (Mugen wannabe), fuel system is completely untouched.

My car runs EXTREMELY rich. I thought it was the ECU, but when the ECU was in my friends car with an identical setup, his motor ran fine.

I beleive I still have the two wire o2 sensor, could this be the culprit? (thought most VTEC motors need a 4 wire) I don't get a CEL, but I'm assuming that's because of the chipped ECU.

 

Just because it ran fine on your friend's car doesn't mean the chipped ECU isn't at fault. I don't know enough about how it's programmed to know if your O2 sensor is the culprit or not, but it is a good starting point.

 

Mugen ECUs run very, very rich.

 

Well you think it is the O2 sensor? Take the sensor out or just disconnect it and see if there is any difference. You should get a cell and it should run richer with no O2 sensor.

 

That would be true, if he were using an OEM ECU. However, the "Mugen" ECUs are race-designed and usually ignore all but the most serious codes... he may or may not get a check engine light, and his ECU may or may not run richer without it.