It depends on which side of stoich you are, but in this case it is highly unlikely the AFR is above 14.7:1 at idle as it would run like crap.
So, a richer mix decreases the AFR and increases unburnt fuel and hence CO.
Here's a table that relates CO % to AFR.
Nell prefers to idle between 13 & 12.4 : 1. Higher than that and she stalls more and doesn't idle as sweetly.
A friends RS was quite happy to idle at 13.5:1, but tends to stall more. I need to richen it up and raise the idle speed by 100 rpm in an attempt to reduce the tendency to stall.
Every engine seems to have its own sweet spot.