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May 8th, 2022
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Allan G Offline 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:

http://www.triumphrat.net/members/albums/62616-open1mind/1969-bsa-plugs-carbon-20617.html

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.

http://www.aussiefrogs.com/forum/at...65d1414285464-reading-spark-plugs-p2.jpg

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. 500°C. The mixture ring forms at the point the ceramic insulator reaches 500°C and it is the ring of carbon on the plug that has a temperature lower than 500°C 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"

http://www.roost.si/articles/jetting/

http://www.ngkntk.co.uk/index.php/problems-with-the-firing-end/

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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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....


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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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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.

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One last comment from my 51 years of looking at plugs. "Mark" the plug with a pencil or sharpie at 12:00. When you remove the plug, hold the "mark" at 12:00 as it was orientated in the engine. If the dark color is on the exhaust valve side, you may have oil coming through the exhaust valve guide. If the dark color is pointing to the intake side, you may have oil coming down the intake valve guide. if the dark area is at 6:00 o'clock, you may have oil passing by the piston rings. Lead, (Tetraethyl, why hi octane fuel was called ethyl) was a GREAT ingredient in gasoline to lubricate engine valves, rings, and boost octane. Valuable for plug reading to determine engine fuel mix, detonation and performance. Just looking at the tail pipe residue was tell tail of engine performance.
It's still sold in the U.S. for use in off-road vehicles, farm equipment, aircraft, race cars, and marine engines. That was when gas had some BANG. Oh, and how i MISS the smell of burning Nitromethane fuel. Talk about adding horsepower. Cars typically need about 15 pounds of air to burn one pound of gasoline. With nitromethane, you only need 1.7 pounds of air, nitro supplied its own oxygen. There is soo much more to nostalgia, then just fond memories.


keep your "oddies" lubricated, and carry a dime
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Does anyone remember this type of plug cleaning tool on sale back in the 60s/70s, the plug screwed into a tube which contained sharpened steel rods, and by shaking they supposedly would clean the carbon off. This would be UK, but other countries might have had similar.


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Semprini, I remember those plug cleaning tools.
My grandfather gave me one, which still had an old 'Lodge' plug screwed into it, as he lost the original cap.
I can't remember where I got the second from.
I think they worked better if you tipped a little petrol into them to help the cleaning.

The second might still be in the garage somewhere, in a plastic box with a Gunson ColourTune. I have no idea
if that still works well with modern fuel.

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I still have the Colourtune, great for the then SU carbs where the idle mixture setting applied throughout the whole range, and the Gunson's tachostrobe which I use on the A65.


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For those tuning at Daytona watch for dramatic changes in humidity, air temperature and barometric pressure during the day. Mid-Winter weather fronts usually arrive between turn one and two. You could tune in the morning, and then watch a front come in at mid-day. This could change things in a matter of hours. I have made substantial jet changes at Daytona in a single day. From the cool damp morning air in practice, to jetting required for that afternoons race. Most of these changes are made based on previous experience and made without doing a "main-jet" plug check.

Also don't surprised if you come from a track that is several thousand feet above sea level, and typically running in the mid-summer heat, that coming to Daytona mid-winter can require a pretty dramatic increase in the main jet.

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Originally Posted by semprini
Does anyone remember this type of plug cleaning tool on sale back in the 60s/70s, the plug screwed into a tube which contained sharpened steel rods, and by shaking they supposedly would clean the carbon off. This would be UK, but other countries might have had similar.

This one was made in England. Still have the cap, but a few wires are missing. They were used to dislodge crap in small passageways, which is the best use I have found for it.

The Colortune has a confidence inspiring crack around the centre electrode. Can’t say that I’m a fan even without the crack.

Do I sound like a curmudgeon?


[Linked Image from live.staticflickr.com]71F53E86-BBBC-4D99-91E2-C2B3D8D35AC2 by First Last, on Flickr

Last edited by Cyborg; 01/05/22 5:36 pm.
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Thanks, a friend will be delighted when he sees your photo

I never heard of a Colourtune breaking (do sparkplugs ever explode?) and wonder if they were ever tested to destruction. I was always nervous of viewing it down a cardboard tube, which was necessary during daylight.

Think mines the original Colourtune.


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Same here… kinda reminds me of this… only with a whole bunch of stupidly added.



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Good grief, one very lucky man!


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Lucky it was full choke that is.


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