The magneto on your lawnmower operates by using the power from the engine to generate alternate currents and send them to the spark plug. The unit itself consists of a number of parts, including breaking points, a capacitor and rotating magnets -- and any one of them could cause problems. If your lawnmower has been sitting unused for long periods of time, the wire between the magneto and the spark plug could be corroded. There could also be a problem with the flywheel that holds the magnet, so testing the complete unit is necessary.
Spin your flywheel. If the flywheel is too far from the magneto, your magneto won't be able to generate enough currency to send to the spark plugs. Remove the engine cover, locate the kill wire and unplug it first. Measure the distance between the flywheel and the magneto; it is usually set from .010-inch to .012-inch. Run your mower and place a match book cover between the flywheel and the magneto core. If the cover doesn't touch either side, you may have another problem.
Look for missing magnets. Sometimes hitting hard objects during mowing, or trying to perform your own engine repairs, can loosen magnets to the point that they fall off. So check along your flywheel and make sure all of your magnets are in place. If they are, one or more of them may be weak, try placing a piece of metal, like a screwdriver, on each one and testing their holding power. You may need to recharge the magnets.
Perform a spark test. Disconnect the wire from the spark plug and insert a metal screw- driver into the boot of the spark plug, which is the area that holds the spark plug. Holding the plastic handle of the screwdriver, bring the metal area of the screwdriver close to a metal part of the engine, being careful not to touch it. Have someone start the mower. If you don't see a spark between the screwdriver and the engine, the magneto is bad.