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So tell me this,... Am I correct in assuming the reason the A/F went rich at that vertical line is because the secondaries kicked in? I am assuming that because of how the A/F stayed more stable running primaries only.
But also YES. Your observation about the primary AFR staying flat and the AFR with the secondary working does indicate the secondary side needs serious tuning.
The slope from rich to lean at the end suggests too large of a main air bleed.
The rich dip as the secondary starts to open could be too large of a main air bleed, too many or too large "emulsion holes" or secondary "idle circuit". So the first change I should havemade was to reduce the size of the secondary's main air bleed. I've done that, but haven't had a chance to retest. I put "idle circuit" in quotes because these circuits also supply the fuel off-idle. In this sense the name is misleading, and most of us spent too many years not getting that fact. (This is why early in this thread I suggested if you were brave to reduce the primary jets and try driving steady at interstate speeds. Cruising at 35 or 40 mph, most of the fuel is coming from the "idle circuit". I personally think that its best to do that test after establishing best jetting for WOT. Less risky and in many cases best for interstate cruising will be the same as best for WOT. )