heres a little how-to i decided to write up about jetting youre YFZ450. the model i preformed it on was a stock 2007 model. the kit i used was the Dynojet stage two kit but other kits are very similar and can be done exactly the same on your YFZ, and almost every FCR carburated machine.
first off, start with a clean quad, because you never want to drop dirt into the carburator.
turn off youre gas. its located right under the tank on youre left side of the quad as though you where sitting on it.