This sounds alot like what is going on with my sled. At wide open i only hit 7500 to 7600 rpms. Here is a list of things i have checked all ready.
Exhaust valves, springs and bellows look fine
Primary, springs rollers look fine
Spark plugs, mag looks good but the pto is dark, rich. Have tried new plugs but didnt help the low rpm problem.
Tried two different belts, polaris 115 and a ultimax
Nothing I have tried has helped. I sled feels like it is running way too rich and almost flooding out at WOT. It has ran like this since taking it out at the begining of the winter.
Have the stock 68 gram weights in it (short track, low elv.). Last year this setup was right at 8100 to 8200 rpm.
No det or check engine lights come on.