Try Bert's solution. I've seen folks remove the third spring to soften the ride. Seems someone already did that on yours if it had three to begin with.
You might find some longer, lighter energy springs so that as the springs are loaded up when you sit on them, the energy builds to hold it at the same height so as not to bottom out the seat deck to the fuel tank pick up. That would not be a good thing at all.
Good luck,
Max