I installed a 39.5 mm pwk (standard on the kx-500) on my wr 500 with very little trouble (different throttle cable). That said, I really liked what it did for me on my bike (coming from a round-slide mikuni 38mm VM). Jetting seemed much easier (seperate choke circuit), and jets are easy to find. I didnt notice any loss in the low end power of the bike, but I probably wasnt using it all anyway. I would buy the PWK 39 again in a heartbeat if I was experiencing problems with a stock carb, even on my 250. My bike eventually ended up running stronger, and better all around, mostly due to jetting with the new PWK. If you do buy an aftermarket carb, make sure it is the same width (distance between reed and airbox) as the one that you currently run (they make a few different sizes).