I am currently rebuilding my 87 250R and was thinking about putting a 39mm off a CR250 onto it but was wondering if it was too big or if I would see any gains. The motor is stock cylinder bored a few times with the ports cleaned up along with a aftermarket pipe and an open airbox lid. Will I see any gains or will I lose??

Thanks

 

You will lose some bottom end but you will gain top end, and it will run smoother. So if you dont mind losing some bottom end then I would go with the bigger carb.

 

Yeah you will be fine. In my experience the loss of low end was barely noticable. I think I just might have THOUGHT it rather than really lost it.