Yurripean Carbs????

SU?? You're dating yourself, OLD man. Actually, a very efficient carb, self compensating for alot of variables.

In my experience, AFB's and Quadrajets are somewhat self compensating also. The secondary throttle plates are mechanically connected, but don't flow any air until the air valve opens. Once the air valve opens, flow drops again because of the extra area of the open venturis. I would think this would cause a kind of fluttering or oscillation of the air valve, since it is simply hanging in the air flow. Sometimes the flat spot is the transition/opening of the air valve, that is probably not real steady at that point. since there is no accelerator pump on the secondaries, a short term lean condition occurs. Nothing serious, but creates that flat spot. Holley "double pumpers" have an accelerator pump on the secondaries to address this problem, but they're mechanical secondaries. However, the Holley spread bore is an air valve system with a pump.

The carbs are pretty much designed so that the primaries are not overloaded before the secondaries open. The air valve controls the metering rods, so only as much fuel flows into the secondaries as is needed. Often the secondaries are set to run a little fat, which is desirable. Carbs use air bleed jets to control the fuel curve, in addition to jets, emulsion tubes, and metering rods.

Other factors affect full throttle on a marine engine..restrictive exhaust and restrictive flame arrestors. The first thing I'd do is check for total advance of the ignition, then remove the flame arrestors and try it again.

It's quite probable that the engine will make about the same power just on the primaries, since the carb is much bigger than needed. Even on my 302 Fords with 450 cfm Holleys, disconnecting the secondaries only drops the full throttle rpms a couple hundred. I've got manifold vacuum gauges on my boat, and full throttle is still so close to zero I can't read it.