Any suggestions for this conundrum?

For several months I have been working on a problem with my 2000 Sportsman 500. The machine, after working on the clutches & rebuilding the front end, last fall, suddenly, after 10 minutes of running, refused to rev up above 1/3 throttle. After multiple posting’s and suggestions, this is what I have done. New carburetor, checked for worn cam, [perfect]. Did a compression and leak down test. Found a problem with worn valves thereby low compression. Purchased new intake valves, and had the seats ground, and valves hand lapped by reputable shop. No change. Removed flywheel, and checked for timing plate, stator coil shift. It’s OK. Finally, purchased a new coil wire & boot, & CDI box, figuring that it was either not advancing timing, or just limiting the revs, no change. It’s charging OK, and sparking when the flywheel passes by the pick up coil, to be expected because it starts & runs ok up to about 1/3 throttle. Other observations; it runs worse when the air box is installed without the filter. It starts a little hard but runs OK at 1/3 throttle after starting. Summer’s coming, and I only got about 4 weeks before I leave for the summer. Any suggestions would be greatly appreciated.