We started the trouble shooting by pulling plug wires independently. If you pull #1, the engine runs but rougher, etc. When you pull the wire on #3 or #4, nothing appears to change. There is spark from the end of the plug wire over to the tip of the plug.

Our compression tester is pretty simple. You screw the end into the plug hole, connect the gauge and turn it over. We did it with all six plugs out but it was so consistent across the cylinders I ruled that out. I was pretty happy to see that it was so consistent.

I have verified the firing order as 153624.

We did adjust the timing a small amount but it was not off enough to affect the other cylinders.

The only thing that I haven't tried is what DennisM noted. I have not pulled a plug and turned it over to verify the plug is actually sparking. I suppose that could be it. We will try that next.