This here is the key, it is the only way to control the seconday AFR externally, meaning: you need to bend the secondary air blade so that you end up with a smaller openning (to richen things up). Beyond this, you have to take the carb apart and go to bigger jets, some enlarge the holes in the Discharge Nozzle (this is where the fuel in the secondary circuit actually enters the airflow) if the jet actually flows enough fuel but the nozzles can not keep up with the fuel delivery into the airstream.

Take a look at the attached scan out of the Carter TQ Service Manual...most of the MP Engine Book stuff (as it pertains to the Carter TQ) which shows the fuel circuits has this information.

Remember, this is applicable to a truly lean condition on the secondaries, as opposed to the more often encountered "bog" when the spring tension is too light.