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Jetting Question

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I have a 98 WR 400. I am thinking of going from a stock jet with stock exhaust system to an FMF header and slip on exhaust. Will/Should I change out the jets or will it run fine with no jetting changes.

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it certainly wouldnt hurt if you go a step up on your main jet and even your pilot. what temperature and altitude you riding at?

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Well with more are in/out, it runs more lean. So you need to counterbalance that with more fuel to make the mixture of air and fuel richer. it may run fine at your altitude and temperature with the stock jetting. you kinda have to wait and see how it runs with the exhaust on first. but if you're like me, and i'm sure you are, you'll want to be able to ride it hard after you get the pipe on it. so maybe buy a jet kit before hand so youre prepared, although, most jet kits dont seem to come with anything except a set of main jets and needle. i buy my jets individually from the yamaha dealer now as well as needle. but all you may need is a main jet anyway.

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