Did you install the little plastic Holley anti-slosh tube in the metering blocks? I know it's more for acceleration with the way these carbs are oriented, but it doesn't hurt to have it in there.

You may also have some blockage in your center metering block emulsion orifices. Is your vacuum advance hooked above or below the throttle blades. What's your fuel pressure?

Vacuum advance must be hooked up to manifold vacuum to work correctly. ie the timing retards when manifold vacuum drops( gas pedal is pushed) and advances it's full amount at idle and up to up to around 50 degrees total when under light load at full mechanical advance on the road etc at high manifold vacuum for maximum fuel economy and torque off idle. It drastically improves drivability around town this way.

You can try lowering the float level in the outboards, or disconnecting them from the fuel supply AND linkage to isolating your center carb until you get this nailed down.

I sent my carbs to Ben at Promax. After giving him my engine specs, they added their metering block. That combined with the use of a wide band to dial it in. It's nothing short of amazing how well it runs.