I think the basic problem here is the very large change in capacity of these carbs when going from part throttle to wot at low speeds.
The old carbs(which were also air valve secondary) have the 4 barrels not so different in size.
So, the primary side has more capacity, the secondary side isn’t as much of an increase in total capacity when they’re opened.
Imagine if the carbs were staggered Venturi diameters, but reversed from normal.
Big primaries, small secondaries.
Intuitively...... you’d expect that would have no bog issues when engaging the secondaries.
As the size of the secondaries grows in relation to the primary side...... coupled with the carbs overall capacity being way beyond what the motor can effectively use........ getting a smooth, clean activation of the secondaries will become more of a challenge.
This has already been demonstrated in this application.
When the carb linkage is disconnected so one carb can be operated as a single unit, the secondary air door operates properly.
By doing that, you have doubled the air flow through the carb(or halved the capacity..... as the motor sees it......whichever way you want to look at it).
At any given rpm, unless you specifically severely limit the carb capacity, the motor is going to try and ingest the same amount of air through the one carb.... or two of them.
Two carbs don’t flow twice as much into the motor...... they just flow 1/2 as much through each carb than if there were only one carb.
This style of carb is dependant on air flow for proper secondary operation, so cutting the air flow in 1/2, creates a problem/challenge.
As for the jetting leaner for cruise.......
My experience in general is that “jetting” has a pretty noticeable impact at lower throttle settings(even carbs that have big restrictions in the fuel passages and don’t respond to jet changes at high rpm with still be noticeably richer with bigger jets at lower throttle settings).
The area of the jet is larger in proportion to the available air capacity at low throttle settings than at larger throttle settings.
Since there are two carbs, they are open even less than normal for part throttle operation........ and there is reduced air flow.
So, it’s normal to see the jetting be quite rich at part throttle with 2 carbs with jetting thats stock or “normal”.
What you have to watch for is, as the throttle settings get open farther, that there is still enough jet area to match the air flow, and that at higher load part throttle settings(like going down the highway at 3500rpm), that you don’t end up with a lean condition.
It’ll either still be fine, or it won’t.
That’s where the O2 gauge really comes in handy.