Casey : Do not have current info on this Saw , but you may well be correct within the air vent , if so normally a Duck Bill Valve is used numerous saws for this reason . It allows atmospheric pressure in but closes against reverse fuel flow. Perhaps self proclaimed Stihl expert Fish can advise of availibility of a Current Download . Good Luck Dude !

Thanks folks! I'll try to run down the part number. I believe I have one of those fittings on another saw but couldn't figure out how air would get into the fuel tank with a set screw in it. Sounds like I can make one myself?