What does a yellow polaris primary spring actually do? Do you need to rejet when changing to this spring?Does it improve the whole power band or just take off?

 

To start swapping clutch components you need an understanding on how the clutch system works.EVERY part in the clutch effects another part in it.A spring change wont effect your jetting.
If you just want to charge the spring go with a Dalton black/white.This spring will work without changing the weights unless you change the secondary setting.If you use too high of a rate of spring you can put the RPM range of you motor too high and blow the engine.