Vacuum Secondaries

FWIW, the Carter/Edelbrock is a mechanical secondary carb but above it is a air velocity valve designed to open along engine demand. The secondaries always open unless the choke is on or not fully opened. The velocity valve can cause a big if you stomp on it to early and if the engine speed isn’t up to consuming it all.

While they are as super tunable as a Holley, they can perform very well. 750 cfm is fine for the 340.

Springs for the Holley are color coded.