There are a few reasons why small engine ignition coils fail. The most common reason is due to a build-up of carbon deposits on the coil. This can happen when the engine is not properly maintained and the spark plugs are not changed regularly. The other main reason for coil failure is due to a loss of insulation on the wire. This can be caused by heat, vibration, or age. If you don't change your spark plugs regularly, carbon deposits can build up on the ignition coil and cause it to fail. Also, if the insulation on the wires wears out, this can also cause the ignition coil to fail. Lastly, age can also lead to ignition coil failure.