My 2011 250 has a 38mm. I've got a YZ250 JD jet kit and will be dialing it in Friday. Once I get it dialed in you are welcome to try my kit in your bike as the jetting should be very similar. It would only take us 15 minutes to swap out.
Randall suggested trying a 42 pilot, N3EH needle (YZ250) and a 180 main on a 300.
Jeff
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2018 XC300
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