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is a rejet needed on aftermarket exhaust?


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Ive been looking at after market exhausts and header pipes. When I switch it out will I need to rejet? 09 yz250f

The only way to find out is by trying it with the aftermarket exhaust. Generally with a slip on muffler, you're OK with the jetting you run currently, but with a full system you should usually expect to have to rejet your bike to make the most out of it. It's usually a trial and error thing, it might work, might not. Also depends on what pipe you get, but for a slip on you should be fine as is.

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Think of it this way-

Is the pipe you are installing gonna make more power? If so then it needs to be jetted richer to make up for more flow. If your new pipe will be quieter it will most likely be more restrictive and if anything need to be jetted leaner. Most likely not if you are replacing a stock pipe.

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On a raptor 660 just an aftermarter exhaust itself can add 5 or 6 hp with the right jetting,remove the airbox lid and jet for that with a good exhaust your talking around 12 more hp... in this case you have to rejet for sure... raptor 660 are kinda lean to begin with. probaly why rejetiing helps so much

I went from 22.5 pilots to 25s both carbs, a 140 to a 160 on the left carb, and a 145 to a 165 on the right carb... and a lot smaller aftermarket needles.. it felt like a 450.

I guess my point is I had to rejet this bike with so much more air flow thru the engine, the oem airbox is really restrictive on the 660s, remove the lid and its not as restrictive...

Your exhaust can't put anymore airout than it can let in, air coming in is what really determines how much fuel you need.

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