Help! Won't rev!

[OldDirtKDX]

Okay my friends, here's the facts; '04 KX, 40:1 synthetic pre-mix. Jetted spot on for altitude and temps. A few rides ago it started missing slightly at high RPM. It continues to get worse to the point that you can hold the throttle wide open in neutral and it won't rev. clean. Plug black and oily since problem started.
What I've done to date: Changed plug, new fuel, checked all conections on coil, checked timing, looked for signs of left side crank seal leakage, cleaned filter, checked reeds, compression.
I'm stumped! I've got the tools and the ability to do the work, I just don't know where to begin.
Any ideas?

 

[whenfoxforks-ruled]

Right side crank seal, do a leak down test.

 

[uts]

I'm thinking your stator is getting week. Happened to an old kx of mine. Next thing it will be impossible to kick start but it will roll start fine.

Check it out anyway.