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Lehanover@aol.com wrote:
I think the SAG is a result of lead salts fouling of the porcelain insulator, in conjunction with the use of the Kettering style ignition systems. With no leaded fuel there seems to be no limit to plug life. And with 100LL about 30 hours. So it is lead salts related. The mechanism of the failure is open to conjecture.
The two mechanism (lead salts and copper corrosion) are not necessarily mutually exclusive. Lead will dissolve copper (easily demonstrable in a few seconds with any soldering iron), and lord knows there is plenty of heat and pressure in that hellmouth called a combustion chamber to make it happen.
It could be a case where the lead accelerates the corrosion of the copper, while leaving deposits that degrade the ignition system. Pilots that feel they have enough glider time in nominally powered aircraft always seem to choose to change out the plugs, rather than fully exploring the extent of what is going on. Only the very edge of the onset of this problem has been explored.
If the lead is helping to destroy the copper center electrode while at the same time conspiring to put a wet blanket on the spark, would a high power ignition simply allow the plug to work until the center electrode is completely corroded and burned away? IOW, would a low power ignition actually make more sense in this case, as it would allow for a gradual onset of the problem and enable the pilot to fly an RV with the performance of a Cessna trainer until the plugs could be changed out.
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