Talk to dan (acutemp on DRN)
He's said he's getting a 39mm for his KDX.
It is from RB, so it's 'plated' (if I understand Sage's nomenclature..same as 'divided').
Generally, the bigger the carb, the worse the bottom-end, the better the top-end. The divider mitigates this somewhat. You keep the velocity up for the bottom-end, and still have enough volume to make the top-end scream.
Makes your carb kinda like a 2BBL carb.
Think of how lousy your car would run if it had one butterfly in the carb (your DO have a car with a carb, don't you?) that had about a 3" diameter.
I had 1300CFM on my 340 for a few years. It worked fine...but only 'cuz every throttle shaft was progressive.
Anyway...talk to dan about his 39mm. If I recall correctly, he just got it this week????
Oh, and I've ridden with a KTM exc200. The KDX can eat it alive! (Yes, I do have the RB carb mod).