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 | auerbach
Join Date: Sep 2007 Posts: 2168 West of Toronto 
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2008-07-18 155394
My 2-cycle whatever won't start, so finally I pull the plug. I clean it thoroughly (brass wire brush, emory paper, rags, compressed air) and inspect it to ensure there's no oil on it, the gap is correct, and both electrodes are unworn. Re-install. Still won't even burp.
Install a new plug, and it starts right up! So what could have been wrong with the old plug? It sure looked like new, but there must have been an electrical fault. Was there some way I could have tested for that?
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 | kwschumm
Join Date: Feb 2003 Posts: 5764 NW Oregon
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2008-07-18 155396
You could probably measure the resistance from the center electrode to the top where the plug wire connects. If the new and old measure significantly different the plug would have an open, but that has to be a very unusual problem. I've heard that if the ceramic insulator around the center electrode has hairline cracks it will allow current to bleed away from the center electrode weakening the spark. ....
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 | harvey
Join Date: Sep 2000 Posts: 1550 Moravia, NY 
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2008-07-18 155397
Even the best cleaning (unless you have a old sand blaster pot) will leave a film that will short to ground.
Use a ohm meter and check for ground short. Typically much simpler to start with a new plug. ....
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 | earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2008-07-18 155399
If you consider the task the plug has to handle, not to mention heat and vibration especially on a 2-stroke, they are pretty reliable all things considered.
I had a 2-stroke, 2-cylinder Yamaha Banshee quad set up for racing. It revved somewhere around 8000-10,000 RPM if memory serves. Twice I had plugs that the insulator broke but didn't separate from the metal base--craziest thing to look at: at idle you see the insulator popping up and down like a valve. The popping noise is what drew me to it. ....
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 | candoarms
Join Date: Mar 2007 Posts: 1932 North Dakota
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2008-07-18 155401
Auerbach,
I'm guessing that your old spark-plug had a hairline crack in the ceramic insulator.
This is a pretty common problem on portable equipment, such as chainsaws, weed trimmers, or any other hand-held equipment.
The insulator can easily be cracked when the tool is kicked, dropped, bumped against something, etc......which happens quite frequently.
A hair-line crack in the insulator would allow the spark to go straight to the engine block.....or ground. Electricity always takes the path of least resistance.
I carry a spare spark-plug in my chainsaw's toolbox, because I've cracked more than one, and it always happens when I'm furthest from home.
Joel ....
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 | hardwood
Join Date: Dec 2002 Posts: 3583 iowa
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2008-07-19 155403
Spark plugs are kind of am amazing thing in that they now go almost forever in a car or pickup without replacement. last time I replaced plugs was in a Venture van with about a hundred thousand miles. Back before EW was born the gas stations used to have samdblasters that cleaned plugs, one of those would probably be an antique now. Frank. ....
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