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Reading Spark Plug Info, Quote From John Healy #681538 01/16/17 10:25 am
Joined: May 2013
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Allan Gill Online Happy OP
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I found this article written by John Healy recently in answer to a thread concerning pre-ignition. I felt that this info to be valuable and worth adding to the fixed topics. I hope you will find it useful too.

Feel free to add more info to this topic...

Originally Posted by John Healy
The plug shown over on the triumph rat forum is an R44XL. It is the equivalent to a Champion RN5C. The resistor version of the N5C. This is two grades hotter than the recommended N3C. This alone could make the motorcycle ping (suffer a good case of detonation with either seizure of move on to pre-ignition and a holed piston) if the bike was lugged!!!!! The incorrect grade of plug is a leading cause of pre-ignition.

It would not be wise to make any diagnosis, or changes until the correct grade spark plug is offered. When you build a house you start with the foundation. In this case it is being sure that you have compression, the factory recommended jetting, valves adjusted and timing set.

Before anyone making comments or suggestions one must go over and look at the pictures! The ceramic core is chalk white. The ground electrode is chalk white from heat. There is a little bit of the grey carbon residue one would typically find on the edge of the steel body. You do not have to log onto the forum just click the link below:

Most of his pictures show little but the third one from the left - top row is telling. Notice that the ground strap is white and the heat soak into the body appears to be at least 6 threads.

Also "reading" a plug requires a lot of experience with the particular engine and fuel being used. Google "spark plug mixture ring" or "jetting ring". This jetting ring is related to teh "self cleaning" temperature of approx. 500C. The mixture ring forms at the point the ceramic insulator reaches 500C and it is the ring of carbon on the plug that has a temperature lower than 500C that we read.
Also "reading" a plug requires a lot of experience with the particular engine and fuel being used. Google "spark plug mixture ring" or "jetting ring"

Because modern fuel has no lead, and that it is the lead that did a lot of the coloring plugs back in the day, spark plugs rarely look like the ones on those old plug pictures copied and posted to the web.

As a side note for those who hunger for that chocolate brown electrode please read Kevin Cameron's comment:
"For plug reading, it is essential that you begin with clean fresh plugs. A dark plug will not lighten to indicate a correct or lean mixture, so used plugs are useless for mixture assessment. Also, forget all the plug manufacturer's four color advice sheets about chocolate-brown. That is the color a plug assumes in a street-driven bike with hundreds or thousands of miles on it. The color you are looking for in main-jet tuning is white. If you make a top speed run of 30 seconds or so, and your fresh plugs come out brown, your engine is hopelessly rich."

Bottom line get the right heat range plug. Verify that the jetting is correct for your model. And make sure the bike is in a good state of tune. Then, and only then, make appropriate changes.

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Re: Reading Spark Plug Info, Quote From John Healy [Re: Allan Gill] #682666 01/26/17 11:03 am
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zxmikez Offline
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Good suggestion to keep those well authored responses alive and permanent. The original thread, and the pics, are from me. I was definitely mis-diagnosing the lean vs rich issue, so again the forum is priceless....

1967 Bonneville
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Re: Reading Spark Plug Info, Quote From John Healy [Re: Allan Gill] #683218 01/31/17 4:05 pm
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htown Offline
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One thing I have noticed with modern gas is that the plugs from newly rebuilt engines and my modern bike all have light gray insulators with maybe just a 1/2 turn to full turn of carbon on the bottom of the threads and clean ground strap with a color change at the bend.
However on my bikes with a quite a few miles on them, I'm seeing more of a brown insulator, more black on the threads and also more black part way up the ground strap. The plugs don't look oily but the black may have sort of a matte texture. I was attributing this to richness but I'm starting to wonder if what I see is due to a slight increase in oil burning due to these engines having more wear. They don't really smoke but I do occasionally have to add a little oil say 1/4 pint every couple hundred miles while I never have to add any to the recently rebuilt and modern bike.
If the darker color is evidence of slight oil burning and not richness, it could be masking a really clean plug signalling a mixture spot on or even lean.
Does anyone have a reliable way to differentiate between carbon fouling(richness) and light oil fouling?

Last edited by htown; 01/31/17 4:05 pm.

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Re: Reading Spark Plug Info, Quote From John Healy [Re: Allan Gill] #734674 05/09/18 10:46 pm
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Dave - NV Offline
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Gents ... It's difficult to read plugs with the modern low lead gas due to the clean burn. But a 'proper' way to read plugs is to use a medical type 'look in your ear' illuminated magnifying instrument that allows you to see the base of the ceramic insulator for the tell tale carbon ring. The position of the ring up/down from the base is a indications of the engines tuning. It's always made me smile seeing wanna be plug readers looking at plugs and giving their 'expert' advise. heh

My first instrument was given me by a local MD friend years ago who had replaced them in the office with rechargeable vs battery powered instruments. Since then some years ago I've bought a few from a supplier for tuner friends and I have a couple extra I'd sell. If you can't clearly see the base of the ceramic insulator you are 'peeing in the wind'.

Above all pay close attention to John Healys explanation noted above on reading plugs and the various references he's given. He 'knows his stuff'.
It ain't no secret why some motor bikes are Fast!

Last edited by dave - NV; 05/09/18 10:54 pm.

Dave - NV

Moderated by  Allan Gill, Jon W. Whitley 

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