I had a problem with keeping the lawnmower running.  It would start just fine, then just die.  Sometimes it would start again but most of the time, it wouldn't.  I checked the carburetor, fuel line, replaced the spark plug but nothing worked.  The problem is the cable had a lot of play (looseness) in it.  After starting the lawnmower, it begin to sputter then I pulled the actual cable and the lawnmower continue to run without dying.  I replaced the cable and everything is working as it should.