Spark plug question....
Tourer
Joined: Oct 2010
Posts: 444
Likes: 166
That is the most enlightening sparkplug reading diagram Iv ever seen.
Is that still accurate with ethanol added fuels?
unobtanium sparkplugs?
Most people think theyre reading plugs just looking at color on insulator
Is that still accurate with ethanol added fuels?
unobtanium sparkplugs?
Most people think theyre reading plugs just looking at color on insulator
Seasoned HDF Member
Joined: Jun 2012
Posts: 6,133
Likes: 2,526
From: CT/FL
@blackjeep I got it when I was racing, about 2009 or -10. It comes from NGK, for their standard spark plugs.
Cruiser
Joined: Jan 2023
Posts: 190
Likes: 409
From: Scotland UK
I used to find my rear cylinder would run slightly richer on my 79 ironhead. No amount of leak chasing, carb jet / spray nozzle adjusting, timing adjustment etc would solve the issue so I used to run a hotter plug on the rear to balance it out. It worked for me and both plugs burned equal colour afterwards.
Grand HDF Member
Joined: Sep 2022
Posts: 4,248
Likes: 3,668
From: Florida
If this chart is right, I’ve been misdiagnosing my timing by the strap for 30 years…. I’ve always (I guess wrongly) understood that what they show as plug heat range on the strap to be a indicator of timing. And what I’ve looked at for plug heat range is how many threads are colored. If more than 4 threads, run cooler.
Can anybody smarter than me confirm this chart is right? So anybody, cause you’re all likely smarter than me lol. An additional note, I hate NKG with the searing heat of a thousand suns. So I’m having trouble with this chart. I run Autolite because for me it’s fact, they consistently can be gapped wider when tested under a full load at high rpm. Other brands cut out a lot sooner than the Autolites. I typically ran 55 or more thou as a gap when at the track. At least 50 on the street. Many really expensive snake oil plugs failed past 45 and if they did get to 50-55 they wouldn’t last but a few passes before starting to cut out n the top end under a load. Autolite just flat did better.
EDITED:
So more confusing, I looked through some books and found info that supports what I’ve been doing. And I also found this on line. It kinda says both methods but adds the detail of tempering your conclusion by studying the porcelain at the same time.
Last edited by Rains2much; Jul 28, 2025 at 06:16 AM.
Seasoned HDF Member
Joined: Jun 2012
Posts: 6,133
Likes: 2,526
From: CT/FL
@Rains2much Your 30 years experience trumps my chart any time. It's all voodoo and black magic to me. There was a guy in our little group who would look at everyone's plugs and I just followed his advice.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,830
Likes: 7,968
From: poway
One thing that bothers me about the Joe12RK one is that I have trouble wrapping my head around the strap/heat range thing.. If the strap changes color at some place, I don't see how it move by going to a cooler plug unless the length of the strap changes. Plugs of the same type but different head ranges have the same strap length.. Maybe heat off the porcelain?
Rains2much's drawing makes more sense..
Rains2much's drawing makes more sense..
HDF Community Team
Joined: Dec 2005
Posts: 5,375
Likes: 2,570
From: NC USA
One thing that bothers me about the Joe12RK one is that I have trouble wrapping my head around the strap/heat range thing.. If the strap changes color at some place, I don't see how it move by going to a cooler plug unless the length of the strap changes. Plugs of the same type but different head ranges have the same strap length.. Maybe heat off the porcelain?
Rains2much's drawing makes more sense..
Rains2much's drawing makes more sense..
The chart with the brown rings is again, for a water-cooled engine but it's as close to what an Evo plug will look like when properly tuned. The top brown ring will never show. That area is where you may see black spots (splattered oil from spark knock) or the tiny silver specks from piston top blasting away from too much timing, too lean, too much heat. You may also see a thin oily black line burned into that area of the porcelain just below the electrode, on the side nearest the intake valve, if you have a bad valve seal.
The two important things to look at are the tip of the electrode for timing and width of the brown band in the bottom for jetting. (detonation evidence will never that deep). Don't worry with the rest of it, it'll drive you nuts.
Always use copper electrodes (no iridium) Whoever said NGX were difficult to read, I agree. I like Champion 404 but "whatever works". Whatever you use, you'll have to ride about 300-400 miles, with minimum cold starts to see all the indicators clearly as shown on the chart with brown bands, Again, only the bottom ring and the tip of the electrode really tell you much for street bikes and it takes some miles before they show up..
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Seasoned HDF Member
Joined: May 2012
Posts: 32,934
Likes: 36,027
From: Carlsbad, CA
Heres an excellent plug reading tutorial from one of the premier race car engine builders. I posted this a while back. He has a better perspective on the strap color and shows it changing with timing.
How to read spark plugs, from an expert - Harley Davidson Forums
https://www.hdforums.com/forum/ignit...an-expert.html
How to read spark plugs, from an expert - Harley Davidson Forums
https://www.hdforums.com/forum/ignit...an-expert.html
Tourer
Joined: Oct 2010
Posts: 444
Likes: 166
Im learning more about reading plugs than ever before.
riding a carbed evo now compared to the higher compression injected bikes before and the plugs are night and day. Amazing how precise a tuned injection system can be compared to a controlled leak carb.
couldnt pay me to go back
riding a carbed evo now compared to the higher compression injected bikes before and the plugs are night and day. Amazing how precise a tuned injection system can be compared to a controlled leak carb.
couldnt pay me to go back