Spark plug question....
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Intermediate
Joined: Mar 2025
Posts: 30
Likes: 20
From: Northern Michigan
That was me.... I was always told the color of the insulator and tip is the key.... Ton of good info here...
Road Master
Joined: Sep 2013
Posts: 1,054
Likes: 441
From: Tejas
Autolite 65 plugs
Been running the AP 65 plugs in both Evo scoots for years and no complaints at all. They work just as well in my 96" as they do in the mostly stock 80" motor.
I'm switching my 98RK to carburetor and swapping out the tank for an 05RK tank and new dash. Also replacing the front fender with the Fatboy style like my dresser. So it's all back from painting the fender and side panels and the new dash is all wired up and working. I'm just trying to get time to assemble the rest of it while I'm remodeling a house 60 miles away and being out there 5 days a week.
Nothing new. LoL
Griz
I'm switching my 98RK to carburetor and swapping out the tank for an 05RK tank and new dash. Also replacing the front fender with the Fatboy style like my dresser. So it's all back from painting the fender and side panels and the new dash is all wired up and working. I'm just trying to get time to assemble the rest of it while I'm remodeling a house 60 miles away and being out there 5 days a week.
Nothing new. LoL
Griz
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,006
Likes: 7,540
From: Poolville
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.
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.
Here’s 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
EDITED: Regardless of technology and fuel, it still comes down to the plugs. And it's the same for twin cams, ethanol fuels etc..
The following pick is a well tuned 110" twin cam with high compression.. The burn of these plugs are almost identical to my 97" Evo...
Last edited by t150vej; Jul 31, 2025 at 09:40 AM.
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