when you use a fater main it will richen up the mid some also.

 

I know this is a pretty old post, but after having a 250R for over 2 years that had to have everything done, I may be of some help.
For a stock Keihin carb 34mm or close to it, 35mm, I suggest starting with 48-50 pilot 150-155 main, needle clip on 2nd or 3rd, air screw 2 turns out. This should put you in the ballpark. Altitude and higher performance parts can make this a challenge. Your local dealership parts guys should let you keep swapping out jets until you get something to work. If nothing works, you may have another problem.