An FMF powercore4 is on the way for my 09 wr450f. Will I NEED to rejet for this exaust?
Short answer is no.
Will it run without a rejet? Yes.
Will it run like crap without a rejet? Maybe.
Will it hurt the motor to run without the rejet? Probably not.
Will it run MUCH better after a rejet? YES!!!!
Needs and wants can be a tricky thing. If you are paying $XXX for the full exhaust bit the bullet and spend the additonal $70 on a JD kit. Comes with a chart that will take 95% of the guess work out of jetting your bike if that is the issue. There are a ton posts and stickys that will help you get it right as well; jetting is wel sorted on this site. If the money for the kit is the issue, then don't buy a full exhaust in the first place and sell the header and keep the muffler only. Full exhaust bonuses are not as huge as it seems: louder isn't always faster.
It probably won't hurt anything but you may actually lose power if you don't.
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