One of the biggest disadvantages of the older 4 strokes vs newer is throttle response. Newer bikes have carbs with accelerator pumps, and once jetted correctly, the extra snap helps immensely with obstacles, breaking the rear wheel loose, etc. My whole plan for this bike was to have it be nimble and snappy, and I knew I needed to ditch the stock carb to accomplish this.
I was planning on getting an FCR, and Sudco listed a kit for a 35mm. But I starting running into a lot of BS about using it. Several people said it was too much carb, though they never tried it. One person who has built several XR250s, and knows his stuff, said he tried it and flat out insisted it would not fit. But Sudco had a kit, so I dug a little deeper, and finally found pics on an Italian site of a 250 with an FCR35 on it. I also never found a single person who actually tried
it. So I ordered it up from Sudco.
Side by side with the stocker:
Side note: while ordering from Sudco, I found out that they have an FCR33mm carb that will fit also. It has a slightly longer fuel-side bell and shorter air-side bell, so it sits about 1 cm farther back. This would be better for cable fitting, but put it closer to the frame rails. Since I was going to increase displacement, I stuck with the 35.