907RotaryFB

So my car runs great at lower rpm but it seems like the secondaries are really hard to get open. When I accelerate and get above 5k there's a substantial bog for a couple seconds and then the engine picks up and revs to limiter. My thinking is that the secondaries are delaying opening but I can't verify that. The secondaries seem to work just fine and the vacuum pot moves freely as do the butterflies. Any ideas what could be causing this?

*edit* Stock 12a engine, no mods, full vacuum rack, carb is totally stock and recently rebuilt with special attention to get the gasket on so it doesn't block the vacuum vent.