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Carbon Arc Tracking in Magneto Distributor Blocks

One common repair for a rough running engine is to replace the magneto points and capacitor. I know from our sales of magneto parts that a very low percentage of distributor blocks are sold along with the points and capacitor. If you believe that the rough running is caused by the magneto, it may be caused by carbon arc tracking between  the distributor block electrodes and have nothing to do with the condition of the points or capacitor. Carbon arc tracking creates a high-voltage short circuit causing the  wrong, or multiple spark plugs to fire.  In the late stages of tracking, burnt carbon tracks appear going from one terminal to another; at earlier stages, carbon tracking is not visible so the block looks OK.

Inside the magneto, conducting carbon dust from the distributor gear brush combines with moisture (or oil from a leaking oil seal) to form a wet conducting contaminate (carbon conducts electricity). This conducting film, along with the large electrical potential difference between distributor block electrodes, causes a current of a few milliamps to flow  through the moist layer causing slight heating. This heating leads to the formation of an occasional very narrow "dry gap" in the conducting film.  Concentrating most of the voltage across the tiny dry gap resulting in  tiny arcs. Such dry gaps and arcs occurs randomly over the surface in an effect known as scintillation (the original Bendix magnetos were called "Bendix Scintilla"). According to research done by The Electrical Distribution Industry in Carbon Arc Tracking: 

These tiny arcs have a temperature around 1000 degrees Celsius and so cause intense heating of the insulation surface on a micro area basis sufficient to pyrolyse (chemically decompose by the action of heat) any organic polymer.

Each tiny arc deposits a  small spot of carbon.  Over time a complete "carbon track" path forms to enable flashover. By the time you can see the "carbon track" your engine has been firing the wrong plugs at the wrong time; a situation that could result in preignition and severe engine damage. 

The best preventative is to make sure you have the latest design distributor block, replace the distributor block regardless of its appearance, and perform regular magneto preventative maintenance.

Severe example of Carbon Arc Tracking damage to magneto When the pilot did the magneto check on run-up, what did this magneto do? Answer: Magneto exhibited normal rpm drop during the engine magneto check prior to takeoff. However, magneto  misfired with a resultant loss of engine power at full power.  As power increases the voltage required to jump the spark plug gap increases. In this example the spark plug would fire at lower engine power settings but at higher power settings the voltage arc'd within the magneto causing the destruction you see on the left.

Normal Bush Wear The carbon brush transfers the high voltage from the coil's secondary circuit to the distributor. As the brush wears small particles of carbon collect on the magneto surfaces. Routine magneto maintenance should include cleaning this carbon dust before it forms a conductive path inside the magneto. The voltage required to bridge a gap is stated by Paschen's Law where the voltage depends on gap distance and gas pressure. V=f(D*P)

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