by crfsonly » Fri May 20, 2011 9:18 am
you are correct about the sizes of each: 250X 37mm, 450X 40mm. these size differences are carefully chosen to match the motor's intake volume. honda moved to 40mm on the 250R and many felt like that was too large for a 250cc motor. it think it absolutely would be for the 250X. the 250X head is significantly different than the late model 250R head. in addition to head differences there are compression ratio and cam differences between the late model R and X models. i doubt you'd be successful with just a carb swap.
what is it you are trying to accomplish?
ken