After reading more about this, one thing that was different between my carb setup and the loaner setup is the carb vent hose. In the set of carbs that ran well, it used a short stock hose that ran from the plastic T connector above the carbs and down the RHS into a red plastic holder.

In my own set, I'm running a very long hose that connects from that T connector down the middle of the carbs, in between the air box boots to where the rest of the drain tubes behind the right foot peg. I wonder if my tube is either too long or getting kinked somewhere and not allowing the carbs to vent and restricting fuel flow?