I went through this dilemma recently with a GEC 78. Got it from a 9 to about a 6/7 in the course of an afternoon. The key was . . . flexing the spring BEYOND it's maximum extension. Leaving the knife open for a while will have a similar effect, but not as quickly and not to the same extent.
In a thread on this very topic a while back, Ken Coats demonstrated that you can either add or subtraction tension from a backspring by flexing it in a special vice. That, of course, required taking the knife apart. But another forumite pointed out that you can do this without taking a knife apart: open the knife roughly 3/4 of the way and insert a small pry bar or similar tool into the space between the tang and the spring. Next, place the butt of the handle and the tip of the blade on a flat surface. Simultaneously torque the pry bar and but down on the knife; the blade will not move, but the pry bar with flex the spring. You only need to flex it an eighth or a quarter of an inch at most past its point of maximal flex to reduce the tension. More than that, and you risk snapping the spring.
I was worried about this at first. I didn't want to damage the knife. But I went ahead with it, and now I can easily open the knife, having done no damage to the spring. I've followed up with WD-40 and a 3-in-1 oil. My American Jack is a nailbreaker no more, about on par with my 71.
Anyway, that's one way to tackle the problem. I can understand if others were hesitant to do this, but I'd happily do it again.
