Small Engines (Lawn Mowers, etc.): B&S 15.5hp Will Not Crank, using jumper cables, lawn tractor

Question
I have a used LT155 White by MTD lawn tractor with a B&S 15.5hp engine that I know nothing about the history of.  I bought it thinking all it needed was a battery to get it running.  The engine will not crank at all with the spark plug installed.  Once the spark plug is removed it cranks just fine.  I have changed the oil and refilled it with the proper amount and I have changed the spark plug.  I've made sure that is no load on the motor by removing the belt and I've also taken the electrical system out of the equation by using jumper cables straight to the starter.  At this I point I am stuck and need some expert help.

Answer
Hi Ben,

If it cranks only with the plug out, I would suspect that either the battery or the starter is weak.  The starter should have enough power to crank the engine when it has compression.  The only other thing that might cause this, is if the ignition timing is advanced.

Let me know how you make out.
Michael