I have an '06 Husqvarna SM610 with a Keihin 41 FCR. The bike runs great except for a midrange stumble that I just can't seem to get rid of. It only seems to happen at steady throttle or trailing throttle between 4k and 5k RPM - on accel everything is fine. It feels as though the bike is misfiring, or otherwise losing power for a split second.
Current setup is 180 main jet, OCEMP needle in the middle(4th) clip position, 58 pilot jet, 100 air jet, about 2 turns out on idle screw, and a Leo Vince full exhaust system.
The only thing I've been able to do that seems to affect it at all is going to bigger and bigger pilot jets, I haven't played with the air jet yet.