Small Engines (Lawn Mowers, etc.): briggs and stratton 14.5 hp riding lawnmower, briggs and stratton, inductive timing light

Question
my mom was mowing the lawn for about 15 minutesa nd it was running strong and then it dies.  She tries starting it up a few times and it back fires.  SO i decided just to replace the spark plug and i took of the head just to check it out.  And the next day it starts right up  Can you tell what elso i can do to prevent this from happening again.  

Answer
Sorry for the delay.  Because it seems to be related to heat and time, I suspect there is either a problem with the ignition module or a problem with valve lash.  Connect an inductive timing light to the spark plug lead and let the engine run.  When the engine starts to die, observe if you lose spark.  If you do, replace the ignition module.  If not, you will need to investigate valve lash.  You should be able to hear a whisping sound through the exhaust if there is an issue with valve lash OR the engine needs the valves lapped in properly (worn or burnt).  Hope this helps.