I just picked up an 04 WR450 w/ a PC 496 pipe, it had been sitting for about a year with the gas still in the carb. I dumped all the gas, removed the carb and cleaned everything out completely then reinstalled it. Filled the tank with fresh gas and it fired right up. A quick rap of the throttle works great, the problem is when riding down the road at about 1/8 throttle, it has a severe stumble then picks up as more gas is applied. Pulled the carb back off and cleaned it again, still the problem. It has a 170 main jet, 45 pilot jet, mixture screw @ 1 3/4 turns out as recommended by PC. Any suggestions?