I did this mod the day I got new tires and the start of our riding season so I don't really know what I gained - one change at a time I guess..
Based off this
by cutting it makes it weeker and takes less vacume to pull the diaphram up#22,the needle is conected to the diaphram and slides in and out of the main jet,by cutting the spring the carb dumps fuel in faster,this is how you get quicker response
I think that your jetting needs to be correct for this to have good effects for a mod. I find that my RPMs don;t build as fast (could be the tires) but also could be the fact that the stock spring had a relationship to the needle and how the user worked the throttle.
Sure the mod may allow more fuel to be dumped in faster but that doesn't mean its being mixed or burned efficiently.
I should have my Quadworks powerkit in next week and I'll post results of going to the DJ spring after jetting and then to the stock cut unit. I know going to 27 inch mudbugs I will lose some snap but I do think the carb spring mod may be flooding too much fuel in which affects what happens off the bottom end.
my .2 , but I don;t know definitively how this mod works yet or if it is just a psychological effect.