Join Date: Oct 2013
Location: Chilliwack, BC, Canada
Let's consider the spark plug gap as most will be familiar with oscilloscope patterns of spark firing. The voltage produced at the spark gap is relative to the resistance of the gap. The higher the resistance, the higher the voltage required to fire the spark/create a high enough voltage to jump the gap. One also notices that the higher the spark voltage required to fire the plug, the shorter will be the spark duration. This is because there is a limited energy stored in the coil's magnetic field (saturated field ignitions) or in the CDI capacitor (CDI ignitions).
Under normal conditions, there is sufficient energy contained in the magnetic field to produce a spark of sufficient duration to provide reliable ignition. (More to this but this simplified example conveys the concepts).
A worn plug requires more voltage to fire so the spark duration will be less. When the spark plug wears sufficiently that the spark required to fire the plug does not allow sufficient remaining energy for a long enough spark duration, misfiring occurs.
One can do the same by taking a new plug and opening the gap to an extraordinary gap and the high firing voltage, reducing duration results in misfire.
That to set the context.
From the example showing that the plug gap both requires increased spark voltage and reduces duration, the following will be seen: of the plug gap is too great for sufficient voltage to develop to begin the spark (ionize the air gap), the spark will not jump despite that the entire magnetic field energy will be involved in driving up voltage.
In an ideal system, there would simply be no spark and the ignition would cycle to the next occurrence. What sometimes happens, especially in an older system, is that the voltage rises high enough to break through the resistance of materials elsewhere in the system. It is not unusual for the spark to jump between adjacent windings of the coil secondary inside the coil which will continue until the arcing burns the wire and then gradually increasing gap will eventually fail to jump. Regardless, if one has pulled the spark plug cap and cranked the engine with "infinite" air gap, the coil can be damaged in this way, eventually to fail later.
It is not unusual for the spark to punch through the insulation on a plug wire or plug cap, to arc down the inside of the plug cap resulting in burning/carbonizing a track which will short the spark.
In CDI systems, the result of leaving a plug cap hanging in the air can be to cause the CDI module to fail.
These are all quite common events which one sees in the profession. That's why one should not crank an engine without either disabling the ignition or providing a ground to the plug lead.
Some will be familiar with the early GM HEI distributors which originally came with black. "blakelite" rotors. If one pulled a plug wire and cranked the engine will the plug lead away from ground, the result was often no spark afterwards. Investigation will show that there is spark from the ignition coil wire and none from the plug wires. This because the voltage developed due to the excessive gap at the disconnected wire, drove the spark voltage high enough to punch straight through the rotor to the distributor shaft.
Last edited by Normk; 06-11-2015 at 06:16 PM.