i have a 4500 with kubota engine. all of a sudden it is super loud like a hole in the muffler and has diminished power with acting like going to backfire. a burning smell like maybe heating the fabric covering the muffler. when I turned it off after limping back to the garage there was a red glow noted inside the muffler. the engine never heated up according to the gauge, since it started doing this just a few minutes after starting to mow. the gas is fresh and plenty in it. I limped back to the garage to put it away and now am writing this thread. could it be a hole in the muffler causing this? somehow changing back pressure? is it running rough and needs a new spark plugs? Is it something else I could handle? I am not a mechanic but would love to be able to fix myself. thanks for any comments? with sincere thanks.