the springs in the variable exhaust close the valve cylinder pressure opens them there is no need to start changing springs thats on all polaris sleds.it sounds like one might not be opening all the way if you can pull 8000rpm and then sqweeze for that last bit and it starts to sputter then that would be my guess. they are easy to fix.
1get 3 spare gaskets, you probably wont need them the first time but in time you will
2there held in by two allen head screws remove the screws and grab the plastic cap and wiggle start pto side it is the easiest, the recoil side you will pull the valve out a little take your finger lift the slide into the cap and it comes out
3 I have had the best luck letting the slide sit in carbon cleaner over night then the next day with a wire brush and some emory cloth they clean nice still a little hard but nice
they get dirty from gas not bad oil or to much oil, more like slow trail riding or breaking trail where you cant open her up, they dont start to open until around 6500 well on a groomed trail 7000 your doing 75
dont pour anything in the engine a cloth soaked in carbon cleaner on a welding rod or small wrench to try to rub it out I have not had any luck cleaning the jugs but good clean valves and good quality gas and a little open strech and they dont need much matienance good luck and it is easier than it sounds
