Why are there no numbers below 4300 or thereabouts? Was the engine run gradually up close to 4200 or 4300 and then opened WOT to make the dyno curve? If so, it could just be a transition condition in the carb (or even in the AFR measuring device(s)). Bottom line: Is this real or just a transition in operating conditions that happens at that RPM due to how the dyno was being run?
If this is for real, (i.e., the dyno pass started with WOT well below 4000 RPM), then the power valve and secondaries with the DP should already be wide open. I would be thinking about the emulsion tube holes not being quite right. As the vacuum in the main venturi's rose with airflow, the level of the fuel in the emulsion tubes gradually drops, and when it drops past one of the emulsion tube holes due to added fuel flow, it can change the mixture a bit. Tuning around such dipsy-doodles in the AFR curve is the biggest reason for the adjustable emulsion tubes. Having said that, I couldn't say which side this is on: primary or secondary.
