Small Engines (Lawn Mowers, etc.): Riding mower Engine will not stay started, briggs stratton, engine cranks

Question
I have a Murray 42" cut riding mower with a Briggs & Stratton 14.5 hp OHV engine. The engine model is  287707 and the Type is 0225 01. Not always ... but often the engine will not strat after using the mower for about 45 minutes. The engine cranks perfectly, the engine starts but runs at a very very low speed for about 5 seconds and stops. It will not restart until about 30 minutes later. The same thing happens occasionally on a cold start. I just replaced the air filter, the fuel filter and a new spark plug. It started right up but woould not start again after cutting lawn for about 45 minutes. Thanks for your help.  

Answer
A couple things come to mind but the first step is isolating the problem.  The next time it refuses to start remove the spark plug and check for spark.  If you have spark then the ignition is fine.  Could be a bad plug.  Have a brand new one ready to install.  Install new spark plug and see if it will start.

If is still will not start with the new plug, remove the air filter and spray starting fluid in the carb to see if it will start.

Trying to determine if there is an igntion or fuel problem.

On a final note, when was the last times the valves were serviced?

Eric