My bet is bad cylinder and rings, but before you tear things apart, check that your oil level is not too high.
Added later: my concern about oil level is based on my having seen engines where the carburetor didn't fully shut off the flow of gas (bad needle valve). The leaking gas ended up in the crankcase, mixing with the engine oil and diluting it until it's very thin. Of course, the leaking gas raises the level of the engine oil, does a poor job of lubrication, and burns.