Spark / No Spark

[jayw]

Found a puzzling problem- after running with little or no load for 5 or 6 sessions of 1 to 3 hours each (in my driveway) the late-model A4 began to miss and run a bit rough. Removing each spark plug wire in turn proved that #4 was not firing. But when putting the spark plug wire back onto the spark plug, the engine smoothed out briefly.
Experimenting showed that if the wire was pushed fully onto the spark plug it would not fire but if the wire was close enough to the plug to jump a small gap, the plug would fire and the engine would smooth out.
The #4 spark plug tip was black and covered with a wet soot, not yet clearly oil-fouled but certainly not clean like all the others. Switching plugs between #3 & #4 solved the misfire, maybe temporarily.
I suspect that the light fouling in #4 indicates some ring problem but the compression is good (112psi) and similar to other cylinders.
Is there any hope that running under load may improve this?
I've already squirted some Mystery oil onto the pistons, But I don't understand why the spark plug works only after jumping a gap to the plug.
Any suggestions would be appreciated, thanks.

[jayw]

After the spark plug swap between #3 & #4 and cleaning the slightly-fouled plug that was put into #3, I launched the boat and ran the engine with moderate to light load for about 5 hours with no problems, other than stalling when pulled back to idle (I'm hoping a carb adjustment will fix). Same plugs & plug wires, engine was smooth.
All plugs looked good, no sign of fouling- so I guess that running under load and hotter has helped.
I'm still puzzled why the slightly-fouled plug would work when the wire was off far enough to cause the spark to jump to the plug but not when the wire was fully pushed on.

[Don Moyer]

I spoke to my friend Tom Stevens about this perplexing issue and his reasoning is the same as mine; i.e., that the additional resistance induced by pulling the lead a short distance from the plug created a slightly higher secondary discharge which was sufficient to break through the slightly fouled condition of the plug.

The reason that increased resistance results in a stronger secondary discharge relates to the analogy of a small stream of water being backed up until the dam breaks, i.e., the higher the dam (equal to resistance in the secondary ignition system), the greater the release of energy will be when the dam finally breaks (equal to the secondary discharge from the coil).

In the case of a small dam break, the rushing water may be stopped by the first row of houses it reaches (equivalent to your partially fouled plugs). In the case of a large dam break, the water will rush right through the row of houses until it reaches a larger obstacle (equivalent to the spark jumping across the center electrode to the grounding electrode of the plug).


Don