Smokin KDX

you didn't say what kind of oil you are running. I had a 01' KDX220 and ran 40:1 with Dumonde Tech oil. I am now a fan of Mobil MX2T synthetic pre-mix at 40:1 in my KTM300. Could be a number of things, leaky tranny, bad jetting, etc. Process of elimination is the name of the game here.
Little Jeff

Well as he said before me, it could be as simple as changing your oil brand. I used Klotz Super Techniplate in my bikes and it seems to be pretty good. I mean you are always gunna have SOME smoke but not excessive. It could also be the tranny fluid leaking into the motor and it is causing it to smoke. Also check to be sure your coolant isn't burning in the motor by checking that your coolant is full and it isn't low. The cause of THAT would be a leaking head gasket letting coolant into the motor.