The only way to see how a plug is working is to take the motor out and run the heck out of it. Immediately shut it down and then check them.
Completely agree here too. However, because this one plug's electrode tip is so different than the other three I feel strongly something has to be going on in that cylinder.
A little background...
Verified with a non-contact infrared thermometer, the starboard side head runs almost 30* hotter than the port side (170* versus 140*). Both thermostat housings are right around 135*.
Both cylinders in the starboard head tested at 140 PSI (wet) while both cylinders in the port head tested 150 PSI (wet). I just bypassed the oil side of the VRO unit and am running a 50:1 premix because I have a non-functional warning horn and may have busted the VRO diaphragm incorrectly priming the oil line.
The engine idles pretty well between 1100 and 1200 RPM.
Before I start replacing fuel pumps and throwing money at guesses I was hoping I could nail down this spark plug. I have never seen a plug get black at the tip like this one.
