Gx620 ignition spark
Have an older gx620 in which started back firing and running rough then quit and won’t start. Put spark tester between plug wire and plug, no spark. Put tester between plug wire and ground and it shows spark. Have installed new coils, plugs, and cleaned magnet. Coils are gapped at .010” flywheel key is good. Grounding wires are disconnected from coils. I am at a loss and ready to replace engine but would rather not. Any help would be greatly appreciated.