Here are some genral clutching rules you should play by!
Primary clutch (Drive Clutch) = Front clutch off the motor
Secondary Clutch (Driven Clutch) = Rear clutch connected to drivetrain
Peak RPM = The RPM the machine sits and holds at most of the time at WOT. This needs to be in your model's peak powerband for best results. BMP Supplies you with your optimum peak RPM when purchasing clutch kits.
More Primary Clutch Weight = less peak RPM
Less Primary Clutch Weight = More Peak RPM
Primary Spring Starting Rate Controls Engagement - Higher the starting rate, the higher your engagement(stall) RPM will be
Primary Spring Ending Rate - Controls Up Shift, Back Shift & Peak RPM
Stiffer Primary Spring Ending Rate = Slower Upshift, Quicker Backshift, More Peak RPM
Softer Primary Spring Ending Rate = Faster Upshift, Slower Backshift, Less Peak RPM
Steeper Helix Angle = Faster Upshift, Slower Backshift
Shallower Helix Angle = Slower Upshift, Faster Backshift
Stiffer Secondary Spring Finish Rate = Slower Upshift, Faster Backshift
Softer Secondary Spring Finish Rate = Faster Backshift, Slower Upshift
Adding horsepower means you need to add clutch weight, go steeper on the helix angle, or both. Most small modifications only require adding clutch weight while larger horspower modifications will require you to change both. Not changing clutching when you add horsepower will result in over-revving.
On some 2 stroke snowmobiles you must add clutch weight or RPM will be too low. 2-stroke pipes are made to work when they are hot. If you do not load the motor hard enough, not enough heat will be built up in the pipe which will result in poor running condition and lower than normal peak RPM. This is about the only case when low RPM is seen, and weight should actually be added.