Check out at a Dollar Tree for the shaving creams. There are some aluminum cans that have no seam in the bottom. Maybe 2 oz. or so. You take them apart from the top and assemble them like that article shows. Sometimes they give more power than open exhaust, which is more than most mufflers. You can open up the exhaust hole a bit on some of the older smaller mufflers if you need a bit more performance. If your idle is bad, maybe it is not something to do, as a bit of backpressure is a good thing.