Small Engines (Lawn Mowers, etc.): Lawn mower wont start, but backfires, Briggs backfiring

Question
I have a Brigs and Stratton lawn mower that is probably 8 or 9 years old.  If I try to start it, it backfires and won't start.  We just replaced the gas line because we thought it was the issue; however, the lawn mower is acting the same after that.  As far as I know I have not hit anything.  Got any idea?  We have replace the spark plug, checked the filter as well as replaced the gas.

Answer
The first thought is usually to see if the flywheel has moved.  If you pull the covers off the flywheel and the retaining nut, see if the flywheel key is still in place and that the keyways line up.

After that I'd need to know if it's backfiring through the carburetor or exhaust.  

Through the carb might be a very lean fuel air mix or a stuck valve.  Through the exhaust usually means a stuck valve if the engine won't run and the flywheel hasn't moved.

Check the flywheel and let me know where it's backfiring through.  Thanks, PK.