i have a 2004 crf450r and i just redid it last week. i did the following... new athena 490 kit, new valves set to spec., dynojet kit, and a K&N air filter. i was wondering what jet you guys would suggest to use, with the standard piston and air filter the jet kit suggested a 150. I put in a 146 because i heard for some reason the athena big bores actually use a smaller size. I may be wrong just wondering. The bike started right up and revs up fine(not too high because of break-in) but backfires after a decrease in RPM. just a pop pop and thats it. just wanting to know why it backfires and if i need to change my jetting.