Small Engines (Lawn Mowers, etc.): 15.5hp briggs stratton wont start, http www youtube, briggs stratton

Question
I have a bolens riding mower with a 15.5hp OHV engine at the end of last year i was driving the mower and the engine got louder and sounded different then popped! it has not started since, it seems as though theres no compression I put my finger over the spark plug hole and there is no pressure, I do have spark. I found a video on youtube of the same engine that wont start. I have added the link so you can see what the engine sounds like when I try to start it, mine is identical! any suggestions would be appreaciated, would hate to throw it away the mower is in excellent shape.
Thank you in advance
Jimmy
http://www.youtube.com/watch?v=Jx00AEO-ftc

Answer
Jimmy
Sounds like you blew a head gasket and probably needs  new rings that's were the compression leaks out first pull the top housing off and check the flywheel key to see if it is not sheared. Then if that is ok you will need to adjust the valves on the engine that will cause a popping sound. There can be a numerous things wrong like a bent valve,bad lifter,bad rings like I said above blown gaskets. You have spark but it is not firing at the correct time to hit the top of the compression stroke. Do check the key way first it that is sheared you will need to pull the flywheel to replace it.