33mm Mikuni Pumper Carburetor
I talked to a few other KLX riders and was advised to first replace the stock CVK carburetor with a decent pumper carb. The 33mm Mikuni pumper carb is perfect for the 250. If you ride a KLX-300, consider the 36mm Mikuni. After securing a super deal on the carb and throttle cable, the next order of business was grinding down the cylinder head motor mounts to accommodate the Mikuni carb's taller slide housing. I removed the stock push pull dual throttle cable system used with the CVK carb and installed a Motion Pro throttle housing. Keep the old throttle cables around though. You'll need them again if you go to the Keihin FCR pumper. An air boot (Air Box side) for a KLX250 allows you to install the carb without making an adapter on the air inlet side -- they can be purchased from Bike Bandit. The carb fits the stock manifold (Cylinder Head side), but it's a sqeeze, so a better solution would be to find a manifold that puts the carb a little further back...
The pumper is taller than the stock CV carb
STOCK CV Carb
So the frame requires some convincing. About 30 minutes of precision grinding was convincing enough
Splitting the carb vent tubes wasn't important to me until I ran through so much mud they became totally clogged, making the bike a bitch to start. This is a simple procedure that carries a lot of value when you need it. You can't see them, but the upper tubes run up into the air box, providing an alternate path in case the lower tube becomes clogged or pinched.