From the factory the stock carburator is set to run too lean with a 148 main jet. If it is not bumped to a 150 you will run hot (you will see the exhaust headers turn blue for half their length). This will cook the exhaust valve seals. You will know this when puffs of blue smoke are produced on backing off the throttle coming into a corner in a race. (blue smoke on acceleration is a sign of worn rings) If you add an after market silencer and K&N filter to a stock bike you will need to go to a 152 main. You should also go up one size on the pilot jet. This will prevent a bog when pegging the throttle from idle. If you modify the motor and port the heads you will need more flow. The stock carb with a 170 jet will work on our screaming 425 but does not flow enough at high RPM. Sparks racing will bore the stock 35.5mm to about 37mm which helps with a mild motor package like a 416. To get the full potential from our full out 425 we went to the Keihn FCR 39mm full race carb. This is the full race version of the YZ426 stock carb. The FCR is recommended by LRD and Sparks. You will need a 160 main and a 50 pilot, with the air screw 2 turns out from closed. This carb really woke up our motor. It has no choke but has a monster accelerator pump. We found that we had to reset the accelerator pump fork adjustment to rid us of an off idle stumble from too much boost. This carb fits the stock intake and air tube and clamps. It is just a little taller than the stock so some truck camper insulating foam strip across the top of the plastic layer under the gas tank will keep the tank from hitting the top of the carb. Unlike the popular Lectron carb, this carb will shut the engine off after a flip and will easily restart without flooding after you roll the bike upright again. This is also a problem with the Elderbrock replacement carb. LRD has dyno tested this carb to be 3hp better than the Lectron across the full rpm band. We use Sunoco 100 race fuel. LRD says our 425 is dyno tested with 93 octane but we don not trust it. A local dealer has the 100 at the pump for motorcycles.