if i added an after market exhaust to my yfz 450, will i have to jet it?

 

It would be a good idea so you don't run the engine lean. Why wouldn't you? You're not going to get the full the full benefit of the exhaust if you don't.

 

Yes. They all run lean from the factory as it is. If the exhaust flows more air, you need to match that with more fuel which will equal more power. The old maxim about a cherry red header means it is lean doesn't really hold for the 450 though. Even the stock header is pretty thin to save weight, and will still glow if properly jetted.