Bicycle Mechanics - Help with My Rear Derailleur, Please

Hi all,

I'm attempting to fix my cousin's bike, the problem:

The rear derailleur is moves out of position when the bike goes to change gears, and the chain is slipping constantly. The problem I can see is the smaller of the two cogs is broken slightly, but I don't believe that to be just the problem. Part of the derailleur goes from being parallel to the tier, to crooked.

The pictures:

The 1328 picture is what the derailleur looks like after the chain slips (obviously).
The 1332 is what the derailleur looks like when the chain is on.

If you need more pictures (which you probably will), just say so.

Any help would be greatly appreciated.

Thanks,
Tom

Very hard to tell from the pictures, but it looks like the derailleur (or hanger) is bent and the axle on one of the jockey wheels is broken or loose. It's never going to shift correctly with either condition.

Looking at the derailleur, you will see two toothed pulley wheels; both are held in in between the "cages" of the the derailleur with a bolt (or axle) in the middle of each pully. The one on the bottom looks okay, but the upper pulley seems to be missing the bolt, or it is broken or has unscrewed. I would start with the this and then see if a bent hanger is also a problem

In layman's terms that means a whole new derailleur, right? I'll take it off, and get a better picture of the derailleur. (In response to sprince).

Severe angle on derailleur cage probably due to a bent hanger. The cage plates at the upper pulley are badly splayed with what appears to be broken edge. Rounded out low limit screw. Bent spoke. Any chance this derailleur went into the wheel?

I would get a new derailuer and see if the derailuer hanger needs to be aligned (shops have a special tool for this).

Any chance the bolt holding the derailleur on is loose?