I've put 6 gallons through an O.S. .12 and stock air cleaner off road before even bothering with the foam. Dirty as hell, but never noticed a change in performance, filter off or on. It just goes to show how these engines are drawing such a tiny amount of air, compared to the displacement of the air cleaner. The dirt is being drawn by the oil, moreso then the engines vacuum.
If our full size cars had an air cleaner of those proportions, half of the hood would be open!
Would you clean that filter every 10 minutes?