Small Engines (Lawn Mowers, etc.): Mower Magneto failure?, spark plug wire, spark plug

Question
Other than not starting, how can you test a Magneto. I have replaced the plug and still no spark at the plug. If I let the mower sit for a day it will start and run for about 10 Min. and then die for another day. The engine is a TECUMSEH 15.5 vertical on a mtd lawn mower. It is a 2004. Plug and filters are new. Today It started up first thing this morning, I mowed about an hour. Went in for a glass of water, came out and again would not start. As it has the last several times it will more than likely start tomorrow.  Any Ideas? I tried taking the ground wire off the magneto but still no spark.  

Answer
The best way to check the ingintion coil is to disconnect the small wire connected to the coil.  You will have to remove the top engine shroud to gain access to the igntion module.

If you disconnect the small wire and have spark the odds are high the coil if fine, 99.99% of the time.

Of course, with the small wire disconnected you will not be able to shut off the engine with the key.  You will have to either choke the engine to shut it off or dis-connect the spark plug wire.

This testing method is the best test for electronic igntion modules/coils, which you have.

Are you absolutely sure the spark plug is not causing the problem.   Are you sure when you come back out you have no spark?
Let me know.
Eric