Nice explanation creep! I will clip 2 and see what it does. It might help my power wheelie quest or it could hinder it. I will find out and post the results. If it sucks I guess I will have to get a new spring. I just like the instant response of my dirtbike. The BST seems like it lags just enough to not get the pop needed for power wheelies....
On the SU carbs did you notice an improvement or a problem by clipping the springs? And have you tried a shorter spring on your 640?