You seem to have done all the "normal " things to sort out the carbs/ mixture.
However, I note that your issues are on 1/4 an 2/3 boundaries. ...Which are also spark coil/ignition divisions. Yes, it could be coincidence.
But, have you reviewed the ignition settings and looked at the spark from each coil on a new set of spark plugs,
A thought... Have you used a new set of spark plugs after making your carb improvements. I note this because a sooted up spark plug can't make a proper spark arc, effecting its self clean ability.
As an objective onlooker, I have to ask if all the rest of the tune up procedures have been addressed before tackling carb fine tuning.
Cam chain tension
Valve clearance
Ignition timing and spark quality
Air filter cleaned or replaced
Compression check
Vacuum sync of the carbs.
Then assuming carbs are really the culprit here:
Have the jets been drilled? They can be "measured" using drill bit shanks.
Are both the main and pilot circuit emuslifiers clear/clean? Are they all the same Keihin brand? (hole size and placement in tube)
Next would be float/fuel bowl level (which you report has been addressed).
Do all the slide needles have the same number on them and appear unworn? Same can be asked of the slide needle jet orifice.
Lastly the carb body circuits.
(Caveat here is that I don't have access to a specific CB750 carb from your bike's era. Design/operation principles are assumed to be the same as a CB550 carb.)
Each emulsifier cavity (Pilot and Main) Has four access points. 1- the jet installation hole, 2. the carb throat delivery hole/exit, 3. the air bleed adjustment screw hole, 4. an air supply jet hole/orifice.
I have found that one has to "prove" the carb internal circuit bores by closing two of the four access points and flowing fluid into the circuit and noting that fluid can enter each point and exit to all the other points in turn. (I recall some forum threads about corrosion blocking some of the passageways.)
This is probably enough to think about for now.
Cheers,
