What makes the CO percentage high on a 1969 VW Beetle?

Hi, i recently went to do a "Vehicle Emission Testing" and it seems the CO percentage is bit high than the previous time. What could make this happen ? What can i do to lower it ?What makes the CO percentage high on a 1969 VW Beetle?Just make sure the points and timing are spot on, and turn the idle air screw in a hair, see if it helps. A dirty air cleaner leads to higher readings, as well