That's a bummer. These things sometimes do get hacked over the years. :(
The heavy spring (aka secondary spring) has a long loop and only engages when the weights are already out 15-20 degrees.
The light spring definately should return the weights easily even though it has a fairly low spring rate.
Halifaxhops might have what you need. But might have to put it on a machine and try a few to get the advance right. I'm sure he'll respond in the AM.