I think that the plug wires are hooked up wrong. I can see in the pic that the first three go to 1234 cyl. That first four on the right side should be 1, 2, 7,3 (pretty sure), I just fixed this same problem. I don't have the firing order in my hand but popping and rough idle would be a good sign of incorrect spark plug wires, especially if the motor waas just pull for the paint. They prolly took off the wires and were too stupid to put them on right. Try that out. Some one give him a firing order please.