Longer version:
The lack of response is probably because most of the fuel has been coming out of the transfer slots. In the first test it was the primary transfer slots, in the second case it was when you opened the secondaries a bit more but closed the primaries.

The large displacement and strong compression helps generate half decent combustion at idle even with that cam. In other words it burns reasonably well and fast compared to many hot rodded engines with lower compression and lots of overlap in the cam.

So I'm guessing that the 11" of vac in gear probably is a little more than QF expected, so it pulls all the fuel needed with just one pair of transfer slots exposed below the blades.

Reducing the timing will generate slightly less vacuum, which should let all the butterflies be where they should. It should also time the burn so a richer mix will generate more effective pressure.

As far as IFRs go. They restrict fuel to both the idle and the transfer slot.
The transfer slot is a restriction to fuel and above the blade is a restriction to air mixing into that fuel. This is why the throttle plate's position relative to the transfer slots has to be withing a specific range.

The Idle Air Bleeds also effect both the idle port and the transfer slot.
Together the IFR and IAB will determine when the fuel from the idle system tapers off as the throttles open up.

One issue with most newer carbs is the IFRs are placed above the fuel level. This can result in less consistant behavior.
(If you find this is an issue, you can relocate the IFR.)

One other possibility is that there is too much bypass air. Bypass is when extra air is allowed past the throttle blades so they can be closed down to the correct initial position. Holley did this by drilling small holes in the throttle plates.
Barry Grant and others used an idea from Bill Jones to create an adjustable 'idle-eze'. Your carb may have that. If so it will say so in the marketing and instructions.
Another way to do achieve the same is to alter the amount of air allowed through the PCV valve at idle vacuum.

In any event, the goal in this case would appear to be allowing less air to bypass if the carb has any of these provisions to allow extra air.