What makes a plug go bad?

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#1
I have been having some problems with fouled plugs. Mainly my bike is running way to rich and the plugs are fouing within 20 mins. When I take the plug out it is wet and has lots of goop on it. After putting in a new plug in the bike will run again for about 20 minutes. My question is this: After thoroughly cleaning and drying the plug it looks brand new. But when I put it back in the bike it will only run for about 30 seconds. What makes a plug bad after it has been fouled? Cosmetically it looks new? I can't imagine something internal that could go wrong in such a short time. I have had similiar experiences with my street bike. Any ideas ?

And I'm getting new jets to fix the fouling...this has just been perplexing me for some time.

2000 YZ 250
DodgedogJB:think :think
 
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#4
My plugs look more like the wet foul only not that black. The goop isn't oily at all, it feels more like black gas. The plugs are very easy to clean and look new when I wipe them with a cloth. There is just way to much fuel coming in. It also feels like its running really rich, such as not wanting to rev to full until its been help wide open for a bit. I'm gonna fix it tonight. I was just curious as to why the plugs are no longer good after they get dirty once? I don't understand it.
 

MBFTY

Uhhh...
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#5
...

From my experence, I have found that cleaned plugs only really work in old oil burning 4 strokes :D


It sounds like you are just a bit too rich. It could be anything from jets that are too fat to a cracked reed. Check your reeds. Its really easy to do. If thats not it, then start looking at the jetting.
 

BRush

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#6
From NGK's website:

Frequently Asked Questions
Q: What is a "fouled" spark plug?

A: A spark plug is considered fouled when the insulator nose at the firing tip becomes coated with a foreign substance such as fuel, oil or carbon. This coating makes it easier for the voltage to follow along the insulator nose, leach back down into the metal shell and ground out rather than bridging the gap and firing normally.

Many factors can contribute to spark plug fouling. The air/fuel ratio may be too rich as a result of incorrect carburetor adjustment or a poorly performing fuel injection system. Worn piston rings or valve seals may allow too much oil to leak into the combustion chamber, leading to oil fouling. The ignition system may not be performing properly. Prolonged idling or continuous low-speed driving may keep the spark plug from reaching its optimum operating temperature. Using too cold a spark plug can lead to the same problem. Finally, a dirty air cleaner can create a too-rich condition which can lead to fouling.

Fuel, oil and carbon fouling can all be the result of different causes but, once a spark plug is fouled, it will not provide adequate voltage to the firing tip and that cylinder will not fire properly. In many cases, the spark plug cannot be cleaned sufficiently to restore normal operation. Therefore, it is recommended that a plug be replaced once it is fouled.
 
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#7
I have found....

that the fine wire or platninum tipped plugs do not respond to cleaning once fouled, but the standard tipped (cheap plug) can be cleaned and reused.
 
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#8
Thanks for all the help guys. The theory of the build up on the insulator is probably somewhat true. Let me first give my background so people don't think I'm just pulling this out of no-where. I have a Materials Science and Engineering degree with a specilization in Metallurgy. I work with several highly educated Ceramic Engineers and a great deal of my work includes working with high temperature (2500F) ceramic materials. After some discussion with several of these guys we partially disagree with the NGK statements. Yes, when a plug first comes out of an engine fouled, the carbon in the oil on the insulator can provide an easier path to ground resulting in no spark. However, a good cleaning should remove this oil from the surface. Ceramic materials, especially those used in spark plugs are extremely dense and it is very difficult to get carbon to diffuse into the surface of a ceramic at the very low temperatures that these engines are running at. While spark plugs are very easy to change and they are very cheap the fact remains that they should be able to be sufficiently cleaned and returned to operational use provided the plug is in good shape otherwise. I will be doing some testing of these fouled plugs and using various cleaners in the next few weeks. I have access to some pretty nasty chemicals so I should be able to get them sufficiently clean one way or another. I also have some high end electrical testing equipment that I should get some good electrical info from. I'll let you know.
 

MBFTY

Uhhh...
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#9
Thats great...

Thats great! Keep us updated... Inquiring minds want to know.
 

Jaybird

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#10
I can tell you what I've experienced with plugs....
I've only had a couple of plugs that were 'gas fouled' , that usually happens only after a crash. After the bike is restarted, immediately after a crash, it POPS and thats it. Those plugs seem to be dead forever.

Now I've also had some 'cold fouled' plugs....those happen when you get oil on the insulator before you reach proper operating temp. Those plugs can be reinserted after you get the bike jetted properly and running well, and warm. From my experience, a cold fouled plug that gets cleaned off from a properly running engine are fine.

Just my 2 pfennigs.