Hammer spring specs?

My .25 bullpup will not shoot less than 910 fps with 25 gr JSB ‘s and I only get about 25 shots before I fall off the regulator. I have tried backing out my hammer spring till the o ring on the adjuster is completely out and there is only one thread holding the adjuster screw in and it still shoots the same. If I screw it in 1,2,3 or more complete turns, the velocity doesn’t change. This leads me to believe my hammer spring is too long or too stiff and I’m shooting hotter than I want and wasting air as well.

Does anyone know what the length, OD and wire diameter is supposed to be?

Or could something else cause said problem?