The little black knob revolves AROUND the spring itself. If you are twisting the spring one of two things is happening. Either the spring has bottomed out in the knob (end of travel) or it is hanging up and not screwing in and out properly (probably do to a poorly finished inner diameter of the black knob). If you are at the end of travel but still have a heavy pull it's probably related to the second problem above, the recess most likely wasn't finish well or deep enough for the spring to screw in far enough.