what causes a spark plug to wear out?

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Nov 29, 2009
My trimmer started misfiring at idle yesterday and almost got to the point where you couldn't let off the throttle. I put a new plug in it today and it runs like new again. I really didn't see anything wrong with the plug. Usually if it does what I described it's because a piece of carbon is causing it to ground out, but this wasn't the case. Another time on a piece of equipment I was having trouble getting it to restart when hot and that's what happened. Also the electrode had worn down to the point where that little v groove was worn and the gap was probably .050 instead of .030"
It's the erosion of the electrodes. Center electrode and ground electrode. I pull mine out every so often to clean them and re-gap them.
many times that center electrode has a crack in it, and the crack can expand/contract with temp too. almost always a spark plug issue on a two stroke is jetting, unless theres TONS of runtime on it. if thats the case smile and toss a new ngk in there and enjoy the cheap easy maintenance of a smoker. my dirtbikes often go several years on a plug, which can be 60 or more hours of runtime easy.
Also Manganese in the gas will contribute to electrode wear on the spark plug. There is a lot of this in our fuel up here at certain times of the year and it will eat the electrodes good on non-iridium electrodes.
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https://bobistheoilguy.com/forums/ubbthreads.php/topics/3090959/Physic_of_the_Gap_%E2%80%93_Spark_Plug#Post3090959 Your magneto system only has so much high voltage available to cause a spark across a specific gap and the fuel. https://bobistheoilguy.com/forums/ubbthreads.php/topics/3092014/Physic_of_the_Gap_%E2%80%93_Spark_Plug#Post3092014
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If you've ever opened one of these things up you can see how simple they are and how failure can happen. There's much more to spark plug wear than just tip erosion. This center electrode carries the voltage to the tip where it jumps across the air gap to ground. These things run hot, are shaken like a James Bond martini, and usually run for years without so much as a hiccup. For OPE it's a wise idea to replace these simple items every few years regardless of tip wear. These pictures are of a Champion RCJ6Y that was only a year old. The center carbon conductor has failed and the plug is now used as a training aid. The poor guy that owns the blower spent his entire weekend fooling with the carburetor on this thing and he never checked his spark.
Spark plugs in modern engines last for an amazingly long time considering the environment they have to live in. In the days of leaded gas the build up of lead oxide on the ceramic insulator meant that mostly they wouldn't survive much over 15-20k miles. 12.000 mile change intervals were the norm on most cars back in the day. Detonation, bad fuel, oil contamination, or water contamination are all things that can shorten a plug's life. Mostly a change interval of 50-100k miles is quite normal, but it wouldn't be the case if many other factors hadn't advanced along with plug technology, particularly engines consistently kept in near perfect tune thanks to computer controlled fuel injection and ignition timing. Claud.
I believe the standard industry service life is 30,000 miles for regular plugs, 60,000 for platinum tipped plugs and 100,000+ for the iridium type. I have seen completely eroded tips at 30,000 miles and I've changed iridium OEM plugs at 150,000 that looked fine. My personal recommendation to people is to use what it came from the factory with and change them when it is recommended.
Generally speaking, spark plugs seldom fail and when they do, there's usually no physical evidence to explain why. Any plugs that I've had fail looked perfectly fine. The last one to do so was early this year in on of my ATVs. It was an NKG plug with fewer than 3000 miles on it and it looked virtually brand new. Nonetheless, it failed. Spark plugs can be tricky. I've had two of them spark outside the cylinder but would not fire inside. That, it very frustrating. I chased that problem for over an hour before trying a different plug. Since then, I always keep a known working spark plug on hand.
It's not unusual for two stroke spark plugs to develop a very thin conductive coating on the insulator. Leading to erratic operation or worse. Plugs are cheap, I simply throw them out.
Originally Posted By: spasm3
Two strokes fire every rotation. I replace my ope 4 stroke plugs at 100hrs, 50hrs for 2 stroke.
Many 4 strokes do too. It's not like small engines have a distributor that runs off a cam shaft. They have a magneto ignition that runs off the flywheel. The flywheel is driven by the crank, so it fires every time around, IIRC. Many cars did something similar with a waste spark ignition. Each coil fired a pair of plugs. One on plug was in the compression stroke, the other in the exhaust stroke (360 degrees apart.) So the plug fired on every revolution in such systems.
Originally Posted By: javacontour
Many cars did something similar with a waste spark ignition. Each coil fired a pair of plugs. One on plug was in the compression stroke, the other in the exhaust stroke (360 degrees apart.) So the plug fired on every revolution in such systems.
Right you are! My olds is waste spark, copper plugs every 25k or double platinum every 50k.
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