What is your normal crusing RPM and what RPM is it that it is richest at? You can restrict the transition fuel feed circuit to lean that circuit out I've done that in the past to help clean carbs. up. The primary jet will lean the crusie AFR out at the RPM above where the throttle blades are opened more than being on the transition circuit at cruising RPMs ( I hope this makes since ) What are the List numbers on your three carbs? Are your carbs originals or the newer replacement -1 or -2 carbs?
I had my old six pak set up to cruise at 14.8 to 15.3 AFR depending on throttle pedal opening at or above 2100 RPM to 2800 RPM
Carbs. are tuneable It does take some knowledge and work though, sometimes including errors to learn what that motor likes
If your idling the motor with .040 transition slot exposed your not idling on the idle circuit only, your already on the transition circuit Fix that first and then proceed