I have been using a Tower .46 engine muffler on one of my 9cc gas engines, and when the muffler came loose in the air, the engine immediately lost power and started misfiring a lot too. I hadn't expected the muffler to affect it quite so much. But it was very noticeable too. So the mufflers that are tuned, or behave like they are tuned mufflers, do have an effect with the engines.