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!