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WR450 exhaust upgrade, know bike back fires even after jet change

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I've been kind of fighting this one as well. I've got a stock YZ exhaust can with a PMB cap. It actually seems to do better with a leaner needle which seems kind of backwards. All of my other 4xx's have been a piece of cake to jet but the 450 has been a bit of a mystery. Anyway I started out going down the richer path (needle & pilot) but that only made it worse. Right now I'm running a JD red #4 and a 48 pilot/100 paj which is pretty good but still pops on occasion. This is not your typical decel popping but more of a snap as soon as you back off the throttle.

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