Its never a good idea to port a muffler with it on the saw! No offense what so ever but its just a v:msp_unsure:ery bad idea. You may think your being careful but metal goes everywhere and inside a muffler, especially one that's not new there is oil residue in which the filings will stick to. And then the saw could be bounced around and they may get dislodged and find there way into the exhaust port. Hey if its your saw its your loss. But its alot better to take it off and port it, blow it out then spray brake cleaner or Carb cleaner into it to loosen up any build up and blow out a few more times until clean. Sometimes on two piece muffs I wash in soap and water dry and still blow down before install. Just my take on things. Its a few extra steps that will help in the long run....