Small Engines (Lawn Mowers, etc.): B&S engine, push mower, fuel leak

Question
John, purchased a used craftsman push mower, 550 series,a 2009 model so looks brand new. it starts easy but once it warms up about every 3 seconds it idles ruff and light puff of black smoke comes out of the muffler. fuel is fresh. any ideas? Thanks, LS

Answer
This could be a few different issues. I would first inspect the air filter and make sure it is not clogged up. If it is, this will result in the engine running rough due to a rich fuel environment. It also may be a result of trash in the fuel, causing the inlet seat to not seal all the way while running. Most of the time this will also result in a fuel leak , but not always.
The last thing i might suspect is the ignition system.
If all other things i have mentioned are ok, and you have replaced the spark plug and it still does it, then i would replace the magneto.

Note: don't just replace the magneto without assuring all these other things are OK. It is very rare to have a magneto perform this way but i have seen it before.
And in most cases dealers will not take back magneto's if that is not the case.