Was there a build-up of carbon in the muffler when you started the project?
A well-tuned engine, either two or four stroke, has hot enough exhaust to burn carbon out of the muffler when it is running under load. If the carbon is bad enough it might even burn a bit at light load or a fast idle. So, if you still have carbon in the muffler, and you have the right oil to petrol ratio, chances are it will put out blue smoke when you load it up, until the carbon is burned away - which means the first one or two times you mow the lawn.
If the muffler is clean and you have the right amount of oil in the fuel, the exhaust will only smoke heavily if the mixture is too rich. I suggest you watch the exhaust until you've mowed the lawn twice, and if it still smokes then, you need to check the carburetor, and re-check to be sure you have the correct amount of oil in the fuel. However from what you've said it sounds as if you probably just didn't get all of the carbon out when you cleaned the muffler. I would normally rely on burning most of it out, unless it was very heavily built-up and partly clogged.
Please let us know whether it continues to smoke. If it clears up by itself, I'll close the thread.
