Lean mixture is one of the causes of an increase in RPM's. As it gets progressively more lean, eventually it is so lean that the engine stops. I would start by checking the fuel level in the bowls which you can do with the carbs on the bike using a clear fuel line attached to the drain spigot and held up to the side of the carb.
I'll try this, how can I tell if the level is appropriate? The floats are both adjusted properly to 23mm
weekendrider has pointed out that you do not have stock carbs...bummer. You could try removing each bowl very carefully to check the fuel level while there is still gas in the bowl. As a minimum, both bowls should have the same amount of gas. You may be able to do something similar by opening the drain plug and catching the fuel from each carb and then compare the amount. Should also be the same for both carbs.
When you quickly mentioned that the float levels have been set to 23mm, it sounds like you recently did that, so that brings us to the universal trouble shooting axiom..."what did you do to the bike most recently?" because that is usually where the problem lies.