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2013 YZ250F runs poorly mid RPM

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At mid RPM machine misfires badly (feels a little like its at the rev limiter) - difficult to push through to a higher RPM.  Spark plug is black.  We run pilot jets one step richer than stock and aftermarket adjustable fuel screws.


Steps taken - leaned out the needle jet to no avail and returned to stock.  Recently discovered the condition worsens with heat and replaced the stator - also did not help.  Swapped the ignition brain and coil from another machine - also no help.  We are at the limit of expertise on this one.  HELP!


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