most of the CA-55 series mixers have a course fuel adjustment on the inlet side of the mixer. Reset your air mixture screw back to about 3 full turns out from screwed in. Then go to the course adjustment valve on the inlet and adjust this to get your best CO reading. Then you can go back to the air mixture screw and fine tune it.
If unsuccessful then replace the mixer(make sure its the same series as what is on there)
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