My 250x has a JE 80mm big bore kit, fmf full exhaust, k&n air filter with a outerwear and I am running a newer model 300ex carb which from the honda manuals says that it is a 32mm. The stock main jet for this carb is a 122 with a 38 slow jet. All of the manufacturers say that you may need to go up a couple main jet sizes. According to powrolls jetting instructions, you go up one size at a time until you notice hesitation which means a "too rich" condition. I have waited for warm weather to do this because the air is heavier and your quad will run richer in the warm months. My findings are: the slow jet is fine, turned out the idle mixture screw 1/2 more turns, moved the needle down one position (second from the bottom, i tried the very bottom groove but it was too much), and i am currently running a 147 main jet. this is 10 sizes bigger than stock and it runs great with no hesitation and the plug is the right color. just thought this might be interesting to those out there wanting to make sure they are getting the most out of there mods.
Jet sizes
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