A tiller engine coil works exactly the same as any coil on an internal combustion engine. It boosts the electricity from the magneto and sends it to a spark plug. The plug conducts this electricity into the cylinder where the electrode on the plug "sparks." The sparking action combusts the fuel inside of the engine cylinder, sending power to the tiller proper. Testing the coil can be done by virtually anyone with even a little do-it-yourself experience.
Pull the wire off of the spark plug. Grasp the spark plug boot firmly and pull it straight off. The spark plug boot is a rubber sleeve that protects the terminal wire where it attaches to the spark plug.
Remove the spark plug with a spark plug wrench. Seat a correctly sized wrench onto the spark plug hex nut. The hex nut is located about halfway down on the spark plug's body. Once the wrench is seated, turn it in a counterclockwise rotation until it becomes loose. It can then be grabbed by your fingers and twisted the rest of the way out. A ratchet with the correctly sized socket will work equally as well.
Push the wire back onto the spark plug. Ensure that the wire is firmly seated.
Lay the plug down on the tiller. The body of the plug must contact the metal frame while the electrode on the plug must remain free and visible.
Pull the starter rope on the tiller and watch the gap on the spark plug for a spark. The gap is the area between the center electrode and the ground electrode at the tip of the plug. This is best done in the shade or at night. The spark that appears each time you pull the starter rope should be bright and intense. If the spark is dim or not present at all, the tiller's coil is bad and needs to be replaced.