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Reading spark plugs

1162 Views 7 Replies 5 Participants Last post by  RT
I'm using Autolite 3924 plugs in my aluminum headed 454, Holley 780 carb, appx 10:1 compression. Yesterday, I put 2 new plugs in #1&2 cylinders, wound it out to about 60 mph for awhile, then coasted to a stop, shutting the engine off as soon as I could. Replaced those two new plugs with the ones I took out, then headed home. Back home, I cut the threaded base off one of the new plugs and found that the insulator was mostly white with a slightly tan band about 1/8" wide at the bottom. I was expecting to see some black so I could get an idea of where to go with a change in jets, but maybe more running time was needed to see that?
The jets that are in the primaries now are 72's any ideas where to go from here?
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Here's the plug.
Eric, from the videos I've watched, a black band about .100 wide is what we want, but this is tan. Oh, and looking closely, you can see it's probably wider than the 1/8" I described.
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For safety sake, no. I had it up in the rpms, but not WOT.
Today, I tried it again with a larger set of primary jets, 74's. The ones that were in there were actually 71's, not 72'as I posted. The car is running stronger with the 74's, but the plug looks the same as before.
Maybe I'm not giving it enough run time? After switching to a new plug, I ran it about 50 mph, hard on the gas and shut it down about 1/4 mile later.
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