It's fine to use, but it can cause idling issues. If you look at the bottom of the filter you will see it has a hole in it, the foam is simply a seal. The idea was, as the filter plugs up it will create more and more vacuum, this vacuum will prevent the metering diaphragm from traveling as far, which means less fuel will be supplied to the carb jets. In theory this should keep the saw in tune as the filter gets dirty. It's the same principle as Stihl's IntelliCarb, but not executed very well on the Dolmar's part.