Small Engines (Lawn Mowers, etc.): John Deere 185 Starting Problem FC540V, john deere 185, 2k ohms

Question
I serviced my mower this spring and the mower ran like usual (I didn't even mow with it, just hauled limbs).  I went to start it a week later and it was backfiring through the carb.  Thought it might be the flywheel key.  I cleaned carb and replaced the flywheel key.  There was a mark on the flywheel key where it sets in the flywheel groove but nothing major.  When trying to start again it was backfiring through the carb and muffler....  Checked the resistance on the ignition module and was within spec.  The plug wire on the coil is ~18.2K ohms which seems a little high.  Compression is 120 psi and the valves are adjusted at .006in.  Checked the flywheel key again and it looked just like the first key, a mark where the key sets in the flywheel groove....

Any ideas on this one.  Could a weak spark cause the backfiring issue?  Since I was getting spark I assumed a timing issue from the flywheel key but this seems ok.  Of course looking for the worst, I just got a new plug today and will try tomorrow.  I am stumped on this one.

Answer
I'm betting the carb was the original problem. The plug is so fouled it's preigniting out the intake valve. The new plug will probably fix it. The only thing that causes a backfire is an explosion with the intake valve open.
If the new plug doesn't solve the problem you will need to replace the ignitor (that's the little thumb size metal box by the dipstick tube). The ignitor does the job of the points in a battery type ignition system. A bad ignitor will fire the plug early causing the backfire.
I'm betting the plug will fix the problem (common, common ,common ).