Any .21-.28 sized air filter will work fine. 2 stage filters work really good. I use motor saver air filters, the pro-line air filters, and the stock air CEN PRO air filters. All work well. DONT OVER OIL your inner filter. Its a great idea to have a few air filters laying around. One thing I completely didnt respect... ZIP TIE THOSE MOTHERS DOWN TIGHT!!! I lunched an engine because I didnt understand how easily an aggressive roll could rip an air filter assembly off of the carb... All part of learning I guess but zip ties are WAAAAAAAAAAAAAYY cheaper than new engines!