Car is a 350 tpi. Last night i noticed a long blue spark shooting out of my coil right where the plug wire was, it was shooting out of it and running across the coil itself. Car has been stumbling and vibrating a lot too. OK, so i changed the wire. No more spark shooting out ,but car is still stumbling. What is the method for testing hei coil? or is it not the coil?

 

Change the cap & rotor and all the wires, and check the plugs.