Ken, I agree that black smoke almost always means rich mixture, except when it means leaking exhaust valves. I take it there was no black smoke, and it ran properly, until some sudden failure caused it to start smoking. Please confirm that when it does this, the exhaust smells rich - the smell is quite distinctive.
There aren't many things that can happen suddenly to an engine to make it run rich. A jet, flow restrictor or mixture adjusting screw in the carburetor coming loose could do it, or the carburetor could have started to flood. Flooding is the most likely problem. There is an easy way to check. I suggest you do two tests. First, park the mower on level ground, remove the air filter so you can look into the carburetor throat, turn on the fuel, and wait a few minutes. If fuel runs out of the carburetor, or down the throat into the intake manifold, it is flooding. The second test is to turn off the fuel with it running at mid-range speed, so the fuel level in the float bowl gradually drops. If it is rich, it will slowly run better, then run properly for a little while, then go lean and stop. If you do both tests you should be able to confirm (or reject) the theory that it is running rich, and see whether it is due to flooding. If it is flooding, something has gone wrong with your float, float lever, float needle, or float bowl fuel seat.
