Did you pre-load the spring? If so, the plugs are probably fouled. Sleds jetted for winter run REALLY rich in the summer, sometimes so rich that they don't have enough power to move themselves forward. That often results in a fouled plug. You can try jetting leaner, but make sure you jet for temperature/elevation come riding season!
spoke with a friend who had a Polaris Dealership up until a couple of years ago about a Indy 400 that I'm working on and he was saying that the 400's from that era were sometimes tempermental and that they were running the red spring in the primary to raise the engagement to get the engine higher in the powerband to avoid a boggy condition as the clutch begins to engage. You might want to give that a try.