I remember reading about this problem before, but no clear explanation was given on how to solve it. I recently repaired a D6C that was having this problem, which is cause by a missing spring. The unit was sold on ebay with rubber replaced, the seller being unaware the spring attached to the flywheel retaining bracket. The spring probably flied out in the process and the seller did not realise it. The solution is to replace the spring with another one from ebay/AliExpress, with the dimensions indicated in the picture below. Note that the spring that I used was cut from a longer 25mm spring.
Happy to read you were able to restore this unit to a fully functioning one! These are to rare to go to waste.
