What a great project, and awesome wood! The original '73's operating instructions say “There are three springs that may lose their tension by long use, viz., the “main spring,” the spring that holds up the finger-lever, called the “finger-lever spring,” and the spring that holds up the carrier-lever, called the “carrier-lever spring.” Each of these has a set-screw, the heads of which are outside on the underside of the frame. By turning these up the tension can be increased to the necessary strength or stiffness.” This indicates that, unlike modern reproductions which are over-sprung, the spring screws were not tightened all the way on originals. If they are tight, then backing them out some will make a big difference. If you go too far on the screws on the bottom, the carrier will start hanging up and the lever will start flopping. Tighten them back up until it functions properly. Make sure the main spring strain screw is backed out. And you can shim the main spring with a washer between the spring and the lower tang to lighten that one.