I have scanned through the pdf I sent you, and see no reference to the spring shown in your pix. Can the ignition circuit breaker unit be rotated by hand? Is it being held in the full advanced mode by the spring? When you looked under the cap, was there any sort of mechanical advance mechanism (weights that are probably spring loaded but will move with increasing centripetal force?). Or does it look like this bike is supposed to have the manual advance/retard mechanism on it. I do not think the spring is "an additional ground". You should have a cable that does that - item 12 or 12A in Fig. 5F-2.
TQ
TQ