Hi Russ - Is it safe to assume that the mechanism is well lubed and free running, apart from the spring tension? If so, it may be worth looking for some kind of a pre-tensioning control. This is akin to fixing a lazy window shade by rolling it up a bit before hanging it. Most of the big FP shutters that I have seen incorporate some kind of tensioning control. Have you stared at it enough to know if this one does? It might be hiding under a cover plate or plug screw. Also, just curiosity here, what is the condition of the fabric itself? It is a continual source of wonder to me that some old FP's are crispy as Saltines and some are supple.

Good luck,