Small Engines (Lawn Mowers, etc.): Intek Engine Coil, function of wire on back., riding lawn mower, secondary winding

Question
Hi Paul,

I have a Scotts Riding Lawn Mower with a 17 hp Intek engine, model 311777.  Recently, I was mowing my lawn, and the engine just died.  I checked out the wiring, and all seemed good, and ended up replacing the Armature-Magneto.
Stuck it in, gapped it, and the mower fired right up, no problem (except for a rather loud backfire).   
Anyway, I noticed a wire on the back of the Armature-Magneto, that seems to be grounded if the switches are in their default positions.  How the heck does the Armature-Magneto thing work, does it just use the Magnets (to build a field) to generate a spark?  Does the wire on the back just short it out and stop it form  working if the switches are not correct?  I understand how a Coil in a car works, just don't know doodle about these small engines.

Thanks,
Fred.

Answer
The coil on your mower works basically just like your car coil.

The magnet on the flywheel creates electricity in the primary winding in the coil,  when the magnet is gone the "juice" transfers to the secondary winding.  When the high spot opposite the flywheel comes by it dumps the secondary winding to cause the high voltage spark. The high spot is doing the same thing as the points in a car. When  the coil is grounded by the switch or the safeties it absorbs the voltage in the secondary winding.

The "backfire" was really what is called after bang.  The engine keeps spinning after the coil is grounded so the fuel is still pulled into the engine but there is not any spark to ignite it.  This vaporized fuel travels into the pipe where it finds a hot spot in the muffler igniting the fuel. BANG!!!!!

Shut the mower off at full throttle and this won't happen.