The most common ignition system is the magneto ignition system which is kind of like a step up transformer. The most important thing about a magneto system is that there needs to be a small gap between the magnets in the flywheel, and the magneto. The flywheel itself is turned by a pull of the starter. There should be two teeth on the starter so it can turn the flywheel, and when the brake is disengaged the cord will rewind. The gap between the flywheel and the magneto should be no wider than the thickness of a playing card. There are many other ignition systems which would take longer than this instrustable to explain, but as a rule you should check all the wiring and look for any wear or grime that might cause problems. You can easily test the ignition system by touching the end of the spark plug to a grounded piece of metal, and see if there is a spark when you start the engine. Do not hold any metal on the spark plug or you will get a painful zap.