I finally have my 2003 YZ450F rebuilt after buying it with a broken case this winter. It runs pretty poorly and I quickly realized I had reassembled the carb incorrectly. This was the first FCR carb I have ever worked on. I had put a type of mystery jet in the Main Air Jet hole since I could not find any other spot it could possibly go. Naturally this kept the main from working at all. My big question is what the heck is this jet/part and where does it go. It is about 1/2" long with no threads, but a screwdriver slot in the top and a very small hole running through the middle, but exiting out the side, not the end. Sorry for the horrible pic, but my camera cannot focus super-close. I will try again later if you need a better picture to help.