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I haves a freshly rebuilt crf450r engine with a 12.5:1 compression piston but i don't know what size jet's to use in it? Everything else in the motor is stock.

Need some more details. Do you have a slip on or full pipe, elevation, tempurature, air box mods. These all play factors in jetting. Me personally I'm at bout sea level with temps from 40-80*f with a full big gun exhaust and I have a 42 pilot 162 main needle at 3rd clip from the top and screw at 1-7/8 turns. Btw it's an 04 crf 450.