I have a nice blasted Jobey Shellmoore that has no filter and was not drawing properly, it was passing a bristle pipe cleaner fine without and discolouration so I got a round hobby needle file and gently filed around the draught hole a bit and widened it slightly. Now it smokes as it should without the sensation of sucking a blocked pipe. I don't smoke filtered pipes and if I get one that has a filter I immediately remove it, sure it may mean extra cleaning but at least I'm smoking from a clear airway (smoke draught hole)