[Mike V. Florida]

Originally Posted by SSsssteamer

To test whether or not the polarity of the coil is properly connected, with the motor running and a loose spark plug wire arcing to ground and creating a spark. Take a lead pencil and put the lead of the pencil into the jumping arc with an air gap on both sides of the pencil's lead. The lead acts as a capacitor and slows the arc to the point that you can see which way the arc is jumping. The spark will be jumping to the ground, or it will jumping from the ground.... You want it to be jumping to the ground. To reverse the spark's jumping direction, reverse the primary leads on the ignition coil.

Pictures of what Ssssteamer is talking about can be found here.

http://modelabasics.com/Ignition.htm