Small Engines (Lawn Mowers, etc.): Oil change and wont start, briggs and stratton, engine cranks

Question
I have a Briggs and Stratton 17 hp Engine Model 31B707-0026. I have been overseas for 6 months and 12 yr old son tried to change the oil. He added 2 stroke oil, but said he never ran it after. I drained it again and added correct oil and level is good. engine cranks very fast and seems to have no compression. Is there a sensor that could be causing the engine to not run.

Answer
Briggs offers an oil sensor switch for engine but it is up to the mower manufacture to order the option and wire into the mower wire harness.

The compression needs to be checked first.  If the engine does not have compresssion start by removing the valve cover and see if the vales are moving.  Sometimes the valves stick when mower sit.  If valves are fine the get a normaly drinking straw, remove the spark plug and insert the straw in the spark plug hole.  Rotate the engine crankshaft by hand.  You should be able to feel the piston hit the straw.  If so, the rod is not broke.  

So, verify the compression first.

Let me know what you find.
Eric