If the spring is too stiff, there are a couple methods to help with that. You can try to relieve metal from the inner side of the spring, which would require taking the knife apart. Basically you would grind away, usually in a scallop-type shape, from the inside of the spring between the center pin and the tang end of the spring, but usually closer to the center pin than the tang end. That is a tricky method, cause you can significantly affect tension with this method, so alot of try and fit is required.
The other method is to relieve the inner corner of the blade tang...the part that sits next to the kick. It doesn't lighten the spring tension, just the amount of pull needed to get the blade started. I usually do this with jewelers files