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I have a 400ex thats been rebuilt to a 426. I just ordered a hmf slip on a k&n filter. Anyone know jetting if riding at 500 ft above sea level?
 

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Jetting is a very individual thing for each engine. With the increase in displacement, slip on and filter, I would order a set of three main jets larger than stock, and try the middle one first. In other words, two sizes above stock. Same with the idle jet. But be very mindful of overheating. A lean engine will run very hot very quickly. Better to be too rich than too lean, if even for 1/2 hour. Test run it in the dark, and if the exhaust pipe glows, you are too lean. This will burn up an engine very quickly.
 
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