2006 kx 85 idles all day, runs clean and crisp. Problem is just before it hits its peak power, right where it should really sing, it cuts out and bogs. if you let off a little it stops and runs crisp again. its only the peak. Is it starving for fuel, air, an ignition problem? Plug is burning great.
any suggestions would be greatly appreciated.
Changed top end to stock cyl. Namura piston and rings.
Have cleaned carb, Set fuel and air to factory ,
Have tried changing needle settings.
Bike has vforce reeds, Carb is mildly ported, pro circut exhaust
Bike ran fine before cyl and piston change had to change mildly ported bondi cyl as there was small hair line crack on one of the exhaust ports. changed it to stock.

Bike is putting out great compression , CDI seems to be putting out good.

We did unhook the kill switch and seemed to cure it for a weekend or two but is back to the terrible bog. "which was wierd"

I am really stumped on what it could be or what to try next.
Going to try pulling off the clutch an check that crank seal, start it and spray to see if there is an engine increase , seal might be sucking not sure if the seal would cause that or not?

Any kind of help would be greatly appreciated to get my Son back up to speed and happy with his performance again.