2 stroke Kawasaki ks125 HELP!

I'm at wits end ( and wallets end). I've been working on a 74 ks125 since June. Bored cylinder,new piston+rings,new con rod,balanced cam shaft, bottom and top both rebuilt. Starts and idles great,but, under load,at upper rpms (5-6k) she sputters and misses, power loss. The only thing I can think of is that at some time in it's life the timing got advanced to far...any ideas,folks?

You done a leak down test?

Have you set the timing with a dial gauge in spark plug hole, finding TDC and then using service manual specs for timing BTDC? Timing is everything on these old 2 strokes.

Also have you considered your carburetor jetting? Sounds like it could be main jet territory.

I'm sure one of the carb gurus here can offer 2 cents.

Leak down test all good,held pressure like a champ.

I was leaning towards timing- the set screw has been loosened and tightened so many times that it was stripped- just haven't gotten to that piece. I pulled the main jet to check it this afternoon,but it hasn't any numbers marked on it so I'm flying blind with that end of things.