You may be better off trying to work with the stock carb. What are you doing for an exhaust? I found that with my XR100, even with a performance exhaust and K&N filter I only had to go up 1 size on the main jet.quote:
should i run a 28mm?
There is the right way, the wrong way and the british way to do anything!
Having said that, when I look at the performance mods for the XR100, you can get a 24mm flatslide for it, suggested if you do the 120cc kit. I can't remember what the stock CB carb is. I'll look in the barn later this week and see if the XR carb is similar to the CB carb.
You can also have your CB carb bored out, which would save you from modifying the mount.