I just purchased my first dirt bike a few weeks ago. It's an '02 Yamaha WR426f and the previous owner lived at sea level. The bike has a Jardine RT-4 exhaust and FMF Powerbomb header, plus the airbox lid has been removed. I live and ride at 5000-6000' of elevation and have realized that the bike has trouble idling (it often dies) and also difficulty starting when warm despite using the hot start button. In all other situations it runs fine. My more experienced friends all suspect it needs to be rejetted for the altitude so I removed the carb yesterday to see what it currently has in it. The stock jetting specs according to the service manual are: Main jet = 165 Pilot jet = 42 Starter jet = 65 The CURRENT setup is: Main jet = 168 Pilot jet = 65 Starter jet = 38 I am wondering if anyone can tell me what the correct jetting should be given the exhaust mods and the altitude where I ride? If you need any other information, please let me know. Thanks!!!