Well the first step would be to take off the GFI and see if the ground and hot are reversed, and if not there keep going back to what you changed to find out where that problem is comeing from.
I believe I said "Common Wire" for the NEUTRAL. Let's try sorting things out.spark plug, when you say common ground, could you elaborate? The 14-3 incoming wire obviously only has one ground. There are 2 branches from the 14-3 (5 wires total). Which of the two 15A breakers do I tie the neutral to?