Another detail to focus on is the throttle stop screw adjustment if you are only checking CO at idle RPM.
I have seen many cases where the throttle stop screw was adjusted improperly, keeping the mixer throttle plate jacked open too far.
Having the throttle plate open too far allows the engine to breathe some extra fuel through the mixer MAIN fuel circuit in addition to the idle fuel circuit.
It is a balancing act to get the proper throttle stop adjustment AND a workable idle RPM.
In no case should the "idle up" dashpot be used to adjust engine idle RPM.
The idle up dashpot should only be very near to its operating point while not actually affecting movement or repositioning of the throttle plate until it is needed (to keep the engine from dying).
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