Small Engines (Lawn Mowers, etc.): Engine turns over but wont start except with a power-pac, briggs stratton engine, briggs stratton

Question
My lawn tractor is an old Murray but with a fairly new 15hp Briggs & Stratton engine.  It's been starting great until a few weeks ago.  

The problem is this:  When I try to start the tractor it turns over but won't start.  However, when I jump directly to the starter with a power-pac battery, it starts up immediately.  I've replaced the battery, cleaned all the connections between the battery and starter, and replaced the spark plug.  The new battery shows 12.5 volts on the voltmeter.

Help!  Any thoughts?

Answer
 Hello Bob:

 From your Description it Sounds like the Starter May be Dragging/Binding. Take a Jumper Cable Set and Using 1 Lead Jumper from the Positive Post of the Battery on the Mower to the Starter. If the Starter is Turning the Engine Slowly, then the Starter is Dragging/Binding. A Power-Pac has More CCA than Most Mower Batteries. Hope this Helps. I am here to Help if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John