Why does your craftsman 6.0 walk-behind clanks at start up and smokes then runs ok and then stalls?

You may be using old gas that was probably stored too long. You should always put fuel stabilizer in the jug you store your gas and in the lawn mower's gas tank as gas will destabilize after 1 month of storage. Destabilized gas will lead to hard start ups, make the mower run inefficiently, and stalling.