In last 30 years working on Vespa scooters, every so often we would find a rubber seal pushed down over the clutch cover breather. I would always remove them. There have been various attempts to explain this rubber seal over the years, but everyone, myself included, was guessing. Until now.
Taking apart a Vespa for some work, we found an original Piaggio PDI tag, which explains the rubber ring, and also explains that it must come off before running the scooter. It makes sense, as the breather is too allow air to vent from inside the engine when it gets warm.
This is the tag, in multiple languages, warning to remove the ring.
On this particular scooter however, it seems the owner did not follow the instructions - you can see the rubber ring still installed on the breather plug
Click here to see a bigger picture
So if you find this ring on a breather plug - be sure to remove it!