i assume you mean you are playing with the ecu screw to alter the CO%? - you shouldnt really mess with it as it adjusts the fueling in the whole map and can cause problems

do you know what the CO% should be at idle, if not you need to phone GGR with your chip ID

was the head rebuilt too or just the bottom end? it could be very worn guides?!

either that or something has gone wrong in the running in process and maybe the rings on 3 arent doing what they should be

or it could be something silly like a dodgy injector on 3 throwing too much fuel in

you can do a comp test to rule out bottom end, but in my experience they arnt always conclusive