Picked up a HRB215HE. It has an intermittent no spark problem. Installed new spark plug and the same thing happened. I disconnected the kill switch from the coil and still no spark. It sat for a couple of days and then it had spark. Ran it for awhile and now it doesn't spark again. I am thinking it is a bad coil. Those are pretty pricey for a used mower so before I start throwing parts at it, any other possibilities? How about the flywheel, could that be causing the problem?