When the variatuer system on my 1978 50v mobylette fully rises, or "opens up" (the belt is pushed all the way up), one of the ball bearings between the main clutch piece and the movable pulley cheek (the part that moves horizontally) gets stuck in place. This prevents the movable pulley cheeck from sliding back, thus keeping the variateur in a fully open position even at low speeds or at idling. The only way to remedy this is to repeatedly shake the clutch and hit the moped on the ground until it shakes loose out of its wedged position.
Could this wedging of the ball bearing between the two clutch pieces be a result of a warn out bearing (or bearing housing)? Or perhaps something isn't ajusted correctly? Maybe the movable part of the clutch is opening up too much allowing the ball to wedge itself?
Here is a chart and diagram labeling all of the parts of the variateur for reference.