In the body air cleaner for the Vespa Large Frame

Perhaps this problem only plagues a large frame Vespa with the LML reed block engine, but I've been fighting oil-fouling of the stock air cleaner since my engine was kitted.  The throat on the reed block is very short with little length to keep high velocity oil droplets from backing up, the stock reeds (Which don't break hundreds of miles from home like the carbon fiber ones can) have a bit of spit-back.  This spit-back oil is coalesced onto the air cleaner, which restricts air flow.  Umpteen miles down the road after cleaning the air filter, and the bike is running rich again.  My carburetor is tuned to exactly, that it can't tolerate any loss of air.  Even changing the grade of fuel is a noticeable change in performance. 

So the only solution I could think of is to move the air filter out of the stock position.  This gives two benefits.  It leaves space for a velocity stack on top of the carburetor, and also prevents any spit-back from fouling the air filter.

I've seen alternate air box covers which give more space. I've seen filter elements inside the rubber bellows.  I've seen the bellows eliminated, and a pod filter used to suck up hot engine air and dirt from the road.  None of those solutions sounded like a good idea.  I wanted an air filter in the body of the scooter.  The air entrance at the front of the seat isn't the only path for airflow, so using that area isn't guaranteed to be all filtered air.  Right where the bellows attaches to the body is the ideal place to put a filter.  The challenge is that this space is shared with the petcock rod, the oil tank, and the fuel tank.  A soft foam filter with some length and small girth could squeeze in there.

I made a 3D printed flange with a clip to attach to the opening in the body.  I selected a Honda CT90 foam air filter element and closed off one end.  There was so much foam there, that I elected to cut the Honda element in half, and still have more surface area than the stock P200 air filter that I was running.  The open end was glued to the 3D printed flange.

The inside of the body is coated in a foam material, presumably for sound deadening, so a perfect seal isn't really crucial.  It took a little bit to push the air filter foam into the hole, and get it positioned so it was open in the center.

The result is very promising.  The performance is fine for my 20/20 carb.