Mower engine sputtering at regular interval

[olmie75]

I have a basic push mower (Yard Machine 21" w/ 4.5 HP 158 cc Briggs & Stratton engine). Recently, the mower has begun sputtering at regular intervals: Every 15 seconds or so, the engine slows down considerably, kicks out a bit of black smoke, and then returns to running normally (then repeats the sputtering in another 15 sec). It's definitely not the grass, since it does this if you run it standing still. Admittedly, I'm not too technically inclined on mowers. If somecody could explain what I might do to determine the cause and fix it, I would be greatful...

 

[Rivets]

Have you checked for a dirty air filter? This is a fuel problem. Probably either water or dirt in the fuel, if the air filter is clean.