Air filter recommendation

The testing does indicate they (K&N type) let through more of the fine particals.
They have an advantage over paper if you expect the filter to get wet stuff thrown at it. Oil, coolant, mud, etc.
Size for size, it seems they generally are less restrictive. Something that may be important when running in a restricted class where every advantage is worked for its potential.

The filter tops can be a problem with most carburetors because:
- they change the direction of air flow over the bowl vents
- can cause unpredictable flow currents in the entry
I'm not saying never use them, but in general better off with a big round filter (like 14 x 3) and a well shaped base and lid.