Small Engines (Lawn Mowers, etc.): 17.5 hp B & S engine wont start., craftsman lawn tractor, briggs stratton engine

Question
I have a Craftsman lawn tractor with a 17.5 hp Briggs & Stratton engine (model # 31C707, type number 0230-E1). Here is my problem. The last time I mowed with it I when shut it off it backfired. (Very LOUD!) Now when I try to start it all it does is crank several times and backfire. I checked the flywheel key by removing the retaining bolt. Slots appear to line up and key looks good. I have changed the oil, drained and replaced gas with new and replaced the spark plug. Pulled the fuel line off at carb, seem to be getting good gas flow at least that far. I also checked for spark by pulling plug and holding it close to heat shield while cranking engine. I am getting spark. Any suggestions on what else to check would be appreciated. Thanks!!

Answer
 Hello Ron:

 You Cannot Check the Flywheel Key Without Removing the Flywheel. If the Key is Good, then Check for a Burnt Valve. The Valves will Have to Be Removed to Check for this. Hope this Helps. Let me know what Happens, Please.

 Good Luck

 Respectfully

 John