My FX-3 has an impregnated foam air filter in the air inlet of the lower cowling. When I inspected it closely I found that, when lightly pushed in, a gap opened between the foam and the sides of the inlet box. This would have allowed unfiltered air to reach the engine when the filter was pushed back by prop wash or slipstream.
My original filter was crudely hand cut and a very poor fit. The replacement machine cut filter (SK53110-001) is at least a half inch wider and appears to seal far better to the air inlet box.
If you operate an EX-3 or FX-3 in dusty conditions I suggest a careful inspection of the fit of the air filter.