Modern Vespa

Bike 1: Running malossi cases, pinasco 60mm crank, mhr mallossi top end, bgm big box, stock p200 ignition, Polini Venturi, and 24.24er pinasco carb on one bike. (No oil injection, but running oil injector airbox cover with drt spacer)

Bike 2: On another I have a hrs alpha kit bored to fit the mallossi mar kit, custom head from Al, pinasco Venturi, polini box, pinasco flytech, pinasco 24.24er carb. (Was running a flat non injector airbox cover with a dry spacer)-I actually didn't realize the flat cover was meant for the non oil injection airbox

I was having low rpm stumble on bike 2 that was more noticeable with a passenger. Played withbjetting and timing, still was present. It would clear up as I built up rpm, but it irritated the hell out of me.

After looking at the airboxes designed for hrs vortex carbs and connecting this to what I have learned about air cleaner clearance over airhorns on Weber dcoe carbs I realized the flat non oil injection airbox may cause airflow issues over the Venturi. I bought a pinasco airbox cover and the stumble is gone. I think a later airbox cover with a spacer or the pinasco cover allows enough space for uninterrupted airflow across the rpm range while the flat non oil injector cover is to close to the top of the air over disrupting flow, most notable at low rpms.

I think the Stella/T5-syle lids have the necessary "bump" over the Venturi. The clear plastic T5-syle from scootermercato are cheap and being able to see through it makes it much easier to seat the machine screws.

The one I have that works is probably from a Stella. I am suprised at the difference the flat lid vs higher lid made.