Originally Posted by Andrew R Stewart

If the chain were too long the der would be pivoting rearwards, pulling the cage and pulleys away from the cogs not into them. Most likely what's going on is the B pivot spring has either broken or one of its ends has disengaged from its insertion hole. I've seen this on a number of Simplex ders over the years (although not for quite a while) and never found a repair method that stayed. I learned to not take apart the B pivot, or even try to adjust the spring tension as doing so has caused this for me a couple of times. Back then a replacement der was really low cost so wasn't a big deal. If you do move to an Asian made der double check that the plastic shift levers will pull enough cable to have the der shift through its entire range. Some of those older Euro shift levers had small cable barrel diameters and many of the first wave Shimano and SunTour ders wanted a lot of cable pull... Andy.