Originally Posted by
Retro Grouch
Examine the spring that pulls the derailleur arm back. If my memory serves, there are a couple of hooks that the end of the spring attach to. The spring may have come unhooked.
If you decide that you need to replace the derailleur, a Shimano Tourney should serve you well. You'll want the version with the claw that attaches to the rear axle.
There is 2 hooks on mine. Tried both of them. The second tighter one allowed me to go into 2nd to top gear without the chain slipping. Thinking about converting it to a single speed. I never actually shift it but I will be moving again in about 6 months to a more hilly area. Not sure yet.