I know relatively nothing about carbs...... but I’m pretty sure it has nothing to do with RPM, it has to do with vacuum signal. Free-revving a motor is not going to pull a much vacuum. Thus, boosters may not be drawing much fuel.
We need Dom......
Booster start point is controlled by the signal generated by the booster, the amount of emulsion/kill bleed above float level, float level, and the main air bleed size. The same carb on two different engines may start booster flow at different RPM's as the pressure difference may not be the same at the same RPM.
