When a trigger is pulled by the operator it exerts force on the sear. The sear is held locked under tension by the hook(s) of the hammer. This tension originates from the force (or pressure) exerted on the hammer by the mainspring. The mainspring tension (force) is a significant determining factor of the firearm's overall trigger weight. The mainspring is the primary source of tension that the sear and trigger have to overcome in order to release the hammer. By placing a reduced power mainspring in a firearm the pressure required to release the hammer is less and therefore the trigger weight is reduced. The problem with that is; by reducing the hammer tension the force pushing the hammer is also reduced, creating a lighter strike by the firing pin. The good news is ; by replacing you old mainspring your pistol should return to it's old reliability as mentioned before by MarkP. You probably will not notice much of an increase in trigger weight but there will be some increase. You may sacrifice a little trigger pull weight but having the relaibility of ignition is a far better gain.