The ones I removed looked like they had little conical brass sponges in them, sort of like the air mufflers you find on a typical pneumatics valve bank (if you work on industrial machinery, that is...). I broke one open: it is a sort of brass mesh. When I dipped it in acetone, balls of water beaded up and fell out of it (which may have 'fixed' it?) as if the water was adhering to the brass. The other one from one of these bikes would let me blow through it, feeling only a little resistance (and a gas-burned tongue), but fogged up completely and stayed that way for an hour or more afterward. Maybe there was some sort of impurity in the brass? I have also seen quite a few bikes where these filters work OK for street riding, but they definitely slow the flow for WOT operation.