Your muffler may be clogged. Where I am, mufflers seem to attract dirt daubers in the off-season, and their nests prevent the exhaust from exhausting. Oil can also turn to tar in a warm muffler.
Take off the muffler while the engine is cold. Don't misplace or damage any screws or rings in the process. Tap the body on a hard surface to loosen what may be inside. Rinse the muffler out from both directions. On a concrete surface, take a butane torch and begin evenly heating the muffler, allowing the flame to enter the muffler. Avoid burning any screen inside. If there's oil / tar built up, it will ignite and burn off. Repeat the heating until nothing burns off, then allow to cool and reassemble.
I've seen them so gunked up they'll burn for an hour.
Phil, August 2007
