High CO is an indicator of a rich condition. Start with the basics, like when was the last time the carb was rebuilt? Is the float level correct? Has it been modified? Rejetted richer for power? Issues or wear in the power valve circuit? CO numbers that high indicate you are dumping a lot of fuel.
Ignition timing will have little to no impact on CO. If HC's are high, retard the timing to decrease, right around TDC usually produces the best numbers, advancing the timing will make the HC numbers worse. If you have access to a gas analyzer, your CO and O2 numbers will cross or be equal when the air fuel ratio is correct.
John
