I think you might be surprised how little the carbs can be out of time and have it affect the tune up.
I thin before I spent more time chasing this relatively low speed drivability issue I would get the unisync and carb hat and get the carbs timed up.
Once you do that it may change the way it drives.
I wish I knew the science behind what happens when the carbs are out of time but I haven't a clue. At my age I'm probably not going to bother learning it.
I'm just know how important it is.