I learned a lesson today. My scag cub with a 23 kaw on it would not run right. Fouled out a plug so I replaced both plugs. Started to run ok, then it would run on one cyl. back and forth between one and two cyl. Fouled another plug. plug was black, running to rich ? I pulled the air cleaner of and it looked like new. The machine only has 90 hours on it. So I took it to the dealer and they found the air filler was plugged up ????????????. I change the prefilter every morning ( have two prefilters, wash one and let dry and use other) I have my doubt if the frefilter really does much. 75 % of my mowing has been in rain, wet grass, no dust. Don't make sense to me. I'm going to try and put a little bit of oil on prefilter and see if that helps.