Carb freshly rebuilt. Idle adjusted per instructions provided (tight, back out 1 and 1/2, then adjust idle and tweak to peak smooth running) and idle is smooth and in range. Low speed drivability is slightly hesitant but pretty normal. Issue is when transitioning from primary to secondardies under heavy pedal (30 mph), there is a flat-spot and hesitation. The secondaries come on (obvious due to induction noise) but the transition is not smooth at all.

I checked the linkage and it looks normal and the opening seems fine. I checked the lower spring loaded butterfly on the secondary and it moves freely with adequate spring pressure (calibrated hand?). I checked timing and its right on timing. using petronix ignition so not an ignition problem, new plugs, wires, cap and rotor. During rebuild the jets were cleaned well and everything soaked clean. Looked pristine upon assembly.

I am struggling to get the choke to the perfect spot, but that shouldn't effect warm running conditions.

Any ideas what to check next?