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This is a Peugeot UO 8 from the 1970s. My problem is recently when moving to the largest gear the chain slips and eventually it will come off the front crank. When researching the problem it seems like the derailleur on my bike is in a different position than ones I see online (red lines). Is my derailleur broken or just needs adjustment?

There is clearly damage to the two plastic gears in the derailleur too which I figured could also be causing me problems.

derailleur

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  • This indeed looks wrong, not an expert on this type of derailleurs but by looking on some other images online, I suspect the spring that pulls the lower part of the arm backwards is broken and needs a replacement. Maybe anyone with some more knowledge will be able to confirm this
    – Max
    May 20 '20 at 5:47
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    The pulley wheels certainly look damaged. You could buy them separately or just get a new derailleur.
    – Michael
    May 20 '20 at 6:15
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    I concur with @Michael - the lower jockey wheel is completely shot, and you should replace it. Do the top one at the same time.
    – Criggie
    May 20 '20 at 6:52
  • Has OP replaced the chain? It looks as if it is way too long. Instead of broken, the tensioning spring may have become unwound. If I remember well, these derailleurs had a spring in the jockey wheel cage only.
    – Carel
    May 20 '20 at 7:10
  • The jockey wheels need replacing. No surprise on such an old derailer. This is likely a major contributing factor to your problems. May 20 '20 at 12:32
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"when moving to the largest gear the chain slips and eventually it will come off the front crank" By largest gear you mean large front, small rear?

I would check if the chain is worn out, before investigating the derailleur. I am assuming

  • old bike, no one bothered to change cassette and crankset, so gears are at least as worn out as the chain, probably much more (like 2-3 chain changes and no gears changes);
  • that if the chain is worn out, for some reason it slips on the front rather than on the rear cog.

If the chain is ok, then time to investigate the derailleur. Internet may help you, there are a lot of documented restoration projects of old Peugeot with simplex derailleur. Example: image taken from https://peugeotcoursepb12.wordpress.com/

from the site https://peugeotcoursepb12.wordpress.com/ (I do not own or know the owner, just a search and some scrolling on duckduckgo images

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It seems to be hanging at an unusual angle compared to modern mechs, but this could be relatively normal for this model of mech. I'd start by fixing the busted jockey wheels first and see if shifting improves.

Here's a random photo showing a red simplex mech hanging more normally: enter image description here
from

Confusingly there are also photos of Simplex mechs hanging more like yours, in a very vertical shot.

enter image description here

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