Newer 3W ignitions use a hall sensor to trigger the ignition...If yours has 2 magnets in the hub it's one of those...If the cap came off the spark could have had no path to ground, so the spark goes through the sensor and destroys it...Sometimes happens instantly, sometimes the spark goes to ground through the cap and works for a while...With some ignitions the sensor can be tested by turning the ignition off and on..Not sure if 3W works that way...The newer ignitions will spark at 1 rpm with the newer systems, no fast spinning needed...