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Hello! I’ve got an 07 crf250r and it’s had about 15 hours or so of run time on a fresh rebuild, it runs great and has power but recently it’s acquired a bit of a sputter on low to mid range acceleration. I changed the plug to the suggested NGK race plug prior to this issue and it’s helped significantly with hot starting. I checked the plug yesterday and it was a little black on the end which tells me it may be a little rich. I’m looking for suggestions on how to get rid of the sputter if anyone has any info! Thanks!

You answered your own question. Sputtering and black plug is as you said = running rich. Check your jetting. You may also try raising the needle clip one position to lean the mid range fueling.