Correct setup of Huret rear derailleur
The tensioning or retaining spring, the small black loop in the last image, at 4 o'clock on the upper jockey pulley, should be clipped behind the protrusion on the cage that is visible in the top left corner of the red frame on the second image. The spring has probably lost tension from age. Putting the spring on that second notch will increase the tension. (The instructions are also included with the link provided / the pink image has the instructions in French, Flemish, English ...)

The job of said spring is to turn the cage clockwise and to tension the chain via the lower jockey pulley.