There is a easy, free mod that you can do to it. I raced a 92 200 in 92 and modified my stock pipe. Keep in mind that I did this 16 years ago and my memory may be a little fuzzy. Take the silencer apart completely. You will see a bunch of tubes that go into a round center plate. You will notice that the main tubes do not meet up with eachother. I simply cut one end of the main tube off of the round center plate, rotated the assy., and joined the 2 tubes together. You can weld or braze it if you want. But the pipe should slip into the other. If you take it apart you will see what Im talking about. There was a noticeable increase in power with this mod. The KX250 may have a bigger inlet, try searching for a pre perimeter frame 125 silencer, should you go that route.
