hi, i'm using a k&n type filter on my honda, and was looking at these foam type ones....do you recon they're better?
as regards the C15 running lumpy at 1/4 throttle, this sounds like its rich....is the float set a bit too high, or do you need a bigger cut away on the slide perhaps?
as a no cost try it solution ,try dropping the needle, and also the pilot air screw should be 1 1/2 turns out....this would let more air in the pilot mix which might help the 1/4 throttle transition.