Actually I was wrong about the Eddy carb as its the secondary side that uses the extra fuel circuits. Looks like it works like the system many Quadrajets use. I said primary earlier but that was a different carb as when I looked at this again I saw I was wrong as its the secondary side that pulls this extra fuel as the air valve starts to open. Ron
https://www.manualslib.com/manual/42643/Edelbrock-Performer-Series-1403.html?page=7#manual