Having a problem with my 99 YZ400. The carb is acting like there is a manifold leak. I somehow caused it when I tore the carb down for cleaning. I've gone back through the carb 3x. I did forget the washer on the pilot fuel screw that goes between the spring and the o-ring on the first rebuild. Wondering if the o-ring got damaged? When I raise the rpm off idle, it takes 4 seconds to drop back to idle. I checked the intake boot, and sprayed carb cleaner, looking for an rpm rise. No luck. Any ideas?