The circumstance you're seeing is referred to as 'carbon fouling'. Carbon builds a 'skin' along the ceramic surface, and becomes a resistor which 'leaks' current to ground. When the coil primary winding is interrupted, the magnetic field of the coil collapses, causing voltage to build rapidly at the high end of the coil tower, which charges the accordant plug wire ONCE the resistance across the rotor gap is low enough AND the spark plug gap has been breached by corona-effect. Having a carbon track from the electrode center to shell (ground) dissipate's the coil's voltage early, holding it's voltage too low to establish corona (incipient plasma discharge... aka... form a spark) at the plug.
The coil energy, instead of 'snapping' across the gap, bleeds off down the carbon trace.
Consider it like this:
Put a golf ball on a tee, grab your favorite driver, and whack that ball 300yds down a fairway. Now step into the shallow end of your local public swimming pool... set that golf ball down in the bottom of the pool, grab your driver and whack the ball. Won't go far... why?
Because all your swing energy is lost trying to push the club through the water.
When you lift the plug wire, the gap in the wire disconnects the plug (a resistance leak to ground) and prevents the coil current from leaking off UNTIL the breakover voltage of the gap you've made, is exceeded. At that point, voltage across the plug gap goes from zero, to extremely high... even WITH the carbon trace, voltage rises too quickly to 'bleed off', and an arc forms across the plug gap... and you have spark.
Many things can cause carbon fouling to start, but once it starts, it's darned-near impossible to stop WITHOUT replacing the plug. Having too cold a plug, either by the incorrect choice, or having an improper mixture, a lazy valve, leaky rings, bad head gasket, or some foriegn blockage in either the intake or exhaust manifold, a leaky intake/exhaust gasket, or a crack in either, will upset the airflow and mixture enough to carbon-soot a plug.
It is also not uncommon for a bad mixture overall (like, too lean) to cause a cylinder to drop, even without carbon tracking... it's just too lean to ignite well, at the compression and spark energy level available. Lifting the plug wire will often make just enough difference in spark 'heat' to light off a very-lean mixture.
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DiyDave wrote: