I am going to heed strong advise and add a second outlet to the muffler. I had to purchase another muffler though, because adding a second outlet to my existing one wouldn't be the right configuration for the test stand or a plane. My intent is to install an 11mm carb on an enlarged intake and message the cyl. port to match as well. So everything should be up to the task.

Just so I'm crystal clear, because now I'm real nervous about grinding on the exhaust port, after goofing up the case. Is the cutter pictured the type of carbide cutter to use on the port?