Small Engines (Lawn Mowers, etc.): Lawn Mower Wont Run, briggs and straton, carbureter

Question
I've got a Murray self propelled 2&1 mower with a Briggs and Straton 5.5 hp Quantum engine that was given to me.  The previous owner had bought it brand new and ran it for about 15 minutes and it quit running.  He never got it to run again.  I cleaned the carbureter and tried to start it, but the priming bulb won't prime the carbureter.  If I put remove the spark plug and put gas into the cylinder, I can get the engine to start and run...sometimes for a few minutes sometimes much longer.  It seems that the carbureter keeps losing its vacuum.  Any ideas?

Answer
 Hello Craig:

 Send me the Model Numbers Off of the Engine, Please. With these Numbers I can View a Breakdown of the Engine and Carburator and Better Assist you. However, from your Description, the Carburator needs to be Cleaned and Rebuilt. If this is Not the Plastic Carburator, Soak the Carburator Overnight in Cleaning Solution and then Blow Out All the Ports and Passages with Compressed Air. Hope this Helps. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John