Rich mixture combined with weak spark will turn a plug black and "foul" it. It still might fire a little at lower rpm for instance. As rpm increases it becomes more difficult for the high voltage to jump across the spark plug air gap. That leads to incomplete combustion, thus fouling the plug even more! Eventually it quits firing.
Oil fouling is another possibility.
Oil could be from a number of sources, like valve guide seals and/or rings not sealing properly. If the fouled plug can't "fire" the unburnt fuel washes down the cylinder and allows
oil to get in the combustion chamber. Which then leads to more fouling!
Could simply be a bad plug, with to much resistance in the center electrode, or to big of an air gap. Maybe a spark plug wire that "leaks" it's high voltage to an electrical ground source. Valve covers for instance, or carbs, or any place the current can find a path to ground BEFORE it gets to the plug. If the plug don't "fire", see above for what will happen!
Sometimes it's the "air gap" in the distributor where the dist rotor spins around and "distributes" the spark to the varius plug wires. IF the dist cap is bad that could allow the high voltage spark to "jump" to another termina. That would be a mis-fire. that high voltage is GOING TO GO somewhere. If the plug won't fire that voltage WILL find a path to ground and the plug WILL foul.
Look at the center electrode of your plug. When the plug gets wet or "fouls" that high voltage might go to the end and run back down the out side of the center electrode. Because the center part is "wet" or "sooty" the path of least resistance may be to "short" on the side electrode RATHER THAN jump the air gap. A short will result in a mis-fire and once again, foul the plug.
Higher compression ratio's make it harder for the spark to jump the air gap. Rich fuel mixtures have the same effect. Thats why "we" typically run an electronic ignition system, to maximize the high voltage.
How high is it? Back in the day, points and coil system, 15 to 20 volts. Electronic is typically 40-50,000 volts. Some new systems put out close to a 100K volts. As the voltage goes up the possibility of a "leaK" to ground increases. Fire will jump right through the plug wire to get to a ground source! Turn out the lights, make it dark, LOOK at your plug wires with the engine running. You might be surprised to see the "light show" taking place! If so, it will foul a plug!
Consider this: AA fuel dragsters run such a RICH mixture it's almost like compressed "liquid" in the combustion chamber. No ordinary ignition system will "light that off"! I'm not sure of the VOLTS they use but I understand the AMPS are like 40 or so. Darn near enough to actually "arc weld" with! The plugs are burned up in one run.
fouled plug
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