I have had the same problem with the fuel screw falling out of my 07 YZ450F I quick remedy is to scribe a line through the threads on the fuel screw to add resistance to backing out. R&D makes a fuel screw adjustment system called Flex-Jet. Its pretty much impossible for it to fall out. If your fuel screw falls out while riding plug the hole in the bottom of your carb. I balled up a little piece of electrical tape to get me back to the truck. runs rich but better than an air leak!