The secondary air valve on the Performer is like the old Carter AFB, in fact they are virtually the same carburetor. The Thunder series is like the slightly-newer design Carter AVS. Both are fine 1960's technology that still works now. The main difference in the AFB and AVS is in how the opening of the secondary air valve is controlled.
On the AFB, it's controlled by a counterweight, and is not adjustable unless you disassemble the carb and modify the counterweight. On the AVS, it's controlled by a spring, and the tension of that spring can be easily adjusted externally with a screwdriver. It's the same basic idea that Rochester used on the Quadrajet.