Make sure your slipper clutch is adjusted properly as well.
With the body off, hold both back tires and try to spin the spur gear with your thumb. While you are pushing on the spur it helps if you roll the tires backward a bit at the same time.
You should be able to slip the spur gear when you apply a moderate amount of force.
This allows it to slip a little bit when you come down from jumps and bumps when the rears are spinning wide open and then come back into contact with traction.