I have a FMF Powercore 4 muffler coming tommorrow and I am wondering what jet will work best with a new YFZ 450. Will stock be ok? Suggestions are appreciated. Thanks

 

If it doesn't already come with jets, call FMF and give them the elevation you normally ride at. That and any other mods you have done to the intake/engine. They should be able to get you pretty close to perfect over the phone. Most YFZ's are lean from the factory even stock, not jetting up could be disastrous for your engine.