Front derailleur sticking, not shifting. Will soaking in solvent damage parts? I've got an older deore lx front derailleur for a mountain triple crankset. It wasn't shifting properly, and it seems that it's sticking and not returning back to a neutral position when I shift it.  I've tried loosening both limit screws with no change in behavior.  I took it off the bike and tried to move the derailler as if I was shifting, but doing that moves the cage, but it doesn't try to return to the original position.

I'd like to soak the derailleur in a cleaning solvent (such as kerosene or paint thinner) to remove any gunk that may have built up on inner surfaces.  Should I worry about the solvent damaging parts? Is there anything else to try to revive the stuck derailleur?
In some derailleurs such as Shimano Tiagra, a small metal nub against which the spring presses is prone to breaking off. In such case there won't be any tension on the spring which could explain the symptoms you're experiencing.