The CO pot is known as the 'global' mod and works by altering the voltage to the MAP sensor, it trims the fuel table by an overall percentage, hence the comment about it potentially running lean at the top end.

A couple of things to check is the MAP reference host is connected/not split!

The CTS is also a corrective factor for the CO and if not giving the correct input will result in rich running.

You say you have checked the regulator, but what about the fuel pressure?

Another common thing is the reg diaphragm can go, which results in vacuum drawing fuel into the plenum via it's reference hose.

Martin