[1QuickS]

There are two air correction jets in Zeniths, one that retains the emulsion tubes in their wells and one that is pressed into the top of the throttle body. Emulsion tubes and their air correctors adjust fuel delivery along with the main jets for 2500+ RPM. The progression circuit consists of holes in the side of the throttle body bore which are uncovered by throttle plate movement from idle along with the idle jet and its unique air corrector jet; all of which control mixture from idle through 3500+ RPM.

As the progression circuit loses effectiveness the main circuit gains effectiveness and the balance of these two circuits is called transition. By not having an adjustment other than idle jet size for the progression circuit you are left with enriching your transition by selecting a large idle jet or with a larger main jet/emulsion tube. Unfortunately the main jet/emulsion tube path is less effective than working the top end of the progression which is what the air corrector jet for the progression circuit controls.

Zenith air correctors for the progression are insanely large @ 160 compared to a normal Weber IDA size of 110; an area ratio of over 200% larger. These air jets lean the progression fuel delivery as the RPMs rise through the transition phase of carburation. Again, these jets are pressed into the top of the throttle bodies and are not adjusted as per normal Weber carburetors; I assume the jets were pressed in on the Porsche carburetors to control end users from tweeking them but Webers sold to customers directly had the tuneable air jets for the progression since the end applications wouldn't be known.