Trying to help diag a friend's car. Running a 6.0 ls3 heads carb intake with 4150 throttle body. Drivers side running 12.8 air fuel and passenger side 11.8 all normal checks have been done. No intake leak no exhaust leak injectors new and have been changed also all grounds and voltage good.

 

Are you reading each bank separately? If so, try to swap the WB sensors and see what you get.

 

Using 1 wb swapped it from one side to the other. Goes lean right of idle.

 

Found cylinders 2 and 4 are running rich and all other cylinders running lean. Still haven't figured out why them two are running rich.