Posted by John T on December 28, 2012 at 17:44:30 from (216.249.82.117):
In Reply to: Long arcing sparkplug posted by pete hall on December 28, 2012 at 16:09:22:
Professor Jim has a great explanation with which I agree (as we usually do) I will just repeat the concept in my own words.
When oil or excess raw gas or carbon (fouling) accumulates on the plugs ceramic/porcelain insulation, such creates a conductive shorting path for current flow. Normally in a non fouled plug when the coil fires its HV primary voltage begins to ramp up and if theres no bleed off current path (like fouling creates) voltage will rise to maybe 8,000 volts and suddenly theres a high temperature arc of current across the plug gap which ignites the fuel air mixture.
HOWEVER if the plug is fouled when the coil voltage just begins to rise all the coils stored energy gets dissipated as current through the fouling material (I squared R) and it never rises high to a point that theres an arc of curent acxross the gap, the voltage only rises to a relatively low value insufficient to arc the gap since theres that fouling material conducting current right from the get go.
HOWEVER if that shorting path is more out of the circuit due to the added wider air gap (like if plug wire is held away from plug) voltage is allowed to rise to that higher value sufficient to arc current across the gap and she goes ahead and fires.
To reduce fouling one might run a leaner fuel/air mixture (reduces black sooty carbon deposits on plugs),,,,Run the engine at a hotter temperature,,,,,,,,Use a higher heat range plug,,,,,,,,,,,Fix the oil burning problem (rings, valve guides etc)
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