I have a 08 WR450F with almost 3,000 miles on it. The only mods I've did to the bike are the free mods to unkork the bike that you find here on thumper talk. I also modified the stock exhaust. I ride trails that range from 4,000' to 13,000' elevation on a regular basis. The reason for the elevation extremes is because I live in Colorado. I have never touched the jets on the bike and neither did the bike shop where I bought the bike. These are the stock jets. I have never had a problem with the bike. It doesn't smoke black smoke, it doesn't sputter, it only over heats and boils over on parts of trails that are extremely difficult and slow, where you would expect it to get hot. The bike also starts very easily no matter if I'm at 4,000' or 10,000'. I guess my question is why change the jet's? Am I missing out on something? Any help would be apprecited.