Small Engines (Lawn Mowers, etc.): briggs and stratton backfires, briggs and stratton, flywheel

Question
Hello,
I have a pretty cheap b&s Bolens push mower, 5 hp, that i purchased in May of 2007, just recently i started having problems starting it.  Now it won't start at all.  When i removed the air filter and tried starting it, it 'pops' and backfires.  I checked the spark plug, put a new one in.  With air filter on, it makes no attempt at 'turning over' when pulling on cord.  Could it be the ignition coil?  I am not sure if there is supposed to be a gap between the coil and the (forgive me for my ignorance) part that the coil rides against.  I appreciate any info u can give me.
thx

Answer
Paul
First of all it still should be under warranty I would bring it back where you got. Most times these mowers will last if you do reg maint on the engine. So for it to start with problems I would suspect a factory issue.BTW it is called a flywheel where the coil rides against

If you still want to find out what is wrong you can try this

Possible causes:

Carburetor set too lean.
Magneto is not timed correctly.
Valves are sticking