I guess I would start with the basics. Air, Fuel, Spark, and maybe compression. I'm guessing your problem is spark. Tapping on it makes me think there could be a short somewhere. The first things I would probably check would first be my coil wire or in your case the cap with the coil in it to make sure your getting a good connection to your rotor. Then for wear or corrosion on your rotor. Make sure there is no arcing problem from a cracked cap or worn wire. If the engine is not turning over I would probably check for a loose cable first then your starter. Some times you can bang on a worn out starter and get it to work. This may be whats happening when your banging on your dizzy. Hope this helps