"from a cut line underneath that goes to the atmosphere"
Over flow line?
Sounds like one carb is flooding and one is not getting fuel.
I'd id go through the carbs again. make sure slides arent sticking, fuel fills both carbs & fuel shuts off, when floats rise it spec hight & go from there.
2 strokes can be a little touchy under load. the new build may actually fix that and have the power to keep the bike "loose" through the whole turn. sometimes a gearing change can help if your too high up in gear and the tires are loose hitting traction puts a too big load on the motor and slows it down just enough to prevent it from keeping loose.
he can also practice slipping the clutch you usually typically can feel and hear when the bike is loosing that "power" and RPM range to keep loose.
i had to do it quite frequently ice racing on a banshee. and would usually always keep a finger on my clutch in a turn and just put enough pressure when needed to get the RPMS back up quickly.
Just note slipping the clutch can be tricky while you do increase the rpms you are technically putting less power down to the ground which can also cause loss of momentum on a hard turn.
When possible taking wider turns at higher speeds also seemed to better suite my bike and allowed me to maintain a better pace and rpm range / gear. depends on rider and bike of course though.
by no means am I a seasoned racer and been out of the game for quite some time but I've definitely had that happen to me and while I didn't perfect it I did work on some solutions and the ones mentioned seemed to work the best for me.