Small Engines (Lawn Mowers, etc.): Briggs and Stratton 26 hp Engine, briggs and stratton, hp briggs

Question
I hace a Sears Garden tractor with a 26 hp Briggs and Stratton engine.  The tractor is 2 years old.  Suddenly it began running rough with a dramatic loss of power.  I have since replaced the plugs, fuel filter, fuel and air filter with no change.  If I disconnect one of the spark plugs the engine stops sputtering and backfiring.  If I disconnect the other instead the engine barely runs.  Hence my conclusion that one of the cylinders is malfunctioning.  All wires appear to be connected properly.  Any ideas?  

Thanks

Answer
Hi Jim-
About all you can do at this point is drop the fuel bowl and check for water.  If thats ok- you need to take this engine to a good shop so they can perform a proper compression check.  You need to findout if you have a burnt valve or not.  I don't  think you can perform a reliable compression check at home on these engines.  I am most suspicioius of the condition of your valves in case you haven't guessed.  I hope you find water in your carburator fuel bowl.
Lemme know ok?