I rode to Davos with this same engine (in a different Lambretta, mind) with an MB panel filter. Despite the GT being a reed valve, every day mid afternoon, I'd have to stop and clean the filter as it was clogged up and really strangling the engine.
When I came back, I ditched the panel filter and replaced it with a Breathe Sweet. However, it was a Series 2 and I could do that. With a PHBH 30 and the GT manifold I have, the Breathe Sweet won't fit inside my Series 3 panels.
Anyway, that's the history of why I have arrived at this:
