What David said,Mine was tight the day I picked it up!The guy that does mine does a great job,been trying to get him to show me how to do it.he's an older guy and wont be around forever,no luck so far.

I got the fold-down seat in and couldn't resist the temptation to disessemble it. The big challenge right now is to rebuild the seat frame. Springs are all rusted beyond use and actually had pieces/parts falling out when separated from back.
Of course, noone makes new seat frames yet, atleast that I could find. I have found pics of the convertible seat frame, 3/4s of it appear to be the same. Which requires modifying the bottom side to mount into the frame. Am I off base on this?

Maybe buy the new standard seat frame and graft what you have into it and get regular springs etc that will work? Welding that spring steel of the frames is tricky business but can be done.. I've been in the same boat as you with rusty seat frames