Small Engines (Lawn Mowers, etc.): Accidental mix of 40:1 gas in B&S 18 HP OHV Intek, briggs stratton, fuel mix

Question
My son has a Sears 42" cut riding mower with a Briggs & Stratton 18HP OHV Intek engine (with oil pump). Two years old. My problem is that last week while he was on vacation I mowed his lawn. When I added gas, turns out it was his 40:1 gas. It starts perfectly and runs for about 5 minutes then it dies. Wait about 15 minutes and it runs for about 2 minutes and then dies. I have replaced spark plug. I am getting a new fuel filter but when I blow through it it doesn't seem clogged. I suspect carburator. Can't find owners manual for engine. Any help appreciated.  

Answer
 Hello K:

 Yes it is the Carburator. The Oil from the Fuel Mix is too Thick to be Pulled through the Carburator Jet. The Carburator needs to be Removed and All the Passages Blown Out with Compressed Air to Remove the Fuel Mix. Send me the Model Numbers Off of the Engine, Please. With these Numbers I can Locate a Breakdown of the Engine and Carburator for you to View Online. Hope this Helps. I am here to help if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John