hmmm, your jetted down on engine, that jetting should be for 4 degree celcius which is awfully warm out, for snow riding you need to be around 146.3/147.5 jets, depnds on your elevation but you may be lean on the jets and the engine will lose rpm if its lean on top, they like to have fuel. Second possible thing is the secondary spring is not any good, with the gearing and primary spring,helix, you should be over revving. The rear sec clutch is torq sensing meaning when it sees a drop in torq/power it backshifts, while your front clutch is trying to upshift, so your gonna get a loss in rpm and "go flat" feeling.
Easiest thing to do to see if your having a engine issue is to look at the wash on piston tops and look at plugs after a pull, should be cardboard brown and not white. Next thing is if thats all good is increase the rear secondary spring tension, and see if rpm rise on top. I would increase the tension 10 degrees each time and see if the rpm comes up if it does, then:
1.)the rear spring is tired
2.) helix angle too big or
3.) your riding in wet slushy snow and that big lug track will draw down the engine in wet heavy snow.You may need to use a stiffer secondary spring, like yamaha silver at 70-80 if when you get the green wound up to above 90.
I would start with jetting first, looks lean to me but I dont know your elevation yet.
