Ihave used them on my smaller planes when I was flying off of a weed field, not grass but weeds plus an old crop duster field. Istarted using them after I had to break down an engine and found weed seeds and little bits of asphalt inside my case. These were .46 to .60 glow engines. Then Istarted using the heel part of my wifes old stockings over the carb and held them on with O-rings, that worked just as well as the little filters I could buy.
Not all engines like to run correctly with a filter on them butt they did need to be replaced quite often so they do work.
Ifly off of a dry lake bed now and have just started using my home made filters again. Several guys are running the small K&N filters on there gassers and they need to be cleaned once in a while so they are doing there job.
Most the guys don't bother with them and I have been flying forseveral years now withoutthem without problems butwith money being so tightthese days I have decidedit doesn't hurt anything to use one. Ican't affordthem for my gassers but Ican make them for my glowengines. If Ihad the extra money I would have themon all my planes right now. A gas engine is runningover 500 bucks these daysso a bit of protection would be a good investment for me.
They work.