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Discussion Starter · #1 ·
I'm so tired of people seizing their engines because they had lean jetting due to relying on plug reading.
So I decided to do a test that shows how unspecific plug reading is, both reading the color and reading the ceramic ring.
Test- 3 new NGK plugs, 3 original main jets (#70, 75, 80), one long uphill, i killed the engine at the top after each WOT run. 2 runs each.
Engine - 48cc air cooled with 8.2 compression ratio, Mikuni 16mm carb, expansion chamber with holed baffle, using Motul 510 (mostly mineral oil) at 28/1
The #80 jet has 14% more hole area than the #70 jet, and by using my jetting calculator I saw that the equivalent flow are of the combination of needle jet and main changed by 8%. Can you see any difference in color? My naked eye could see the very faintest of increased darkness in the plug that used the #80 jet.
Conclusion- there is no way on Earth that plug reading is specific enough to be useful to select a main jet. That is nothing but wishful thinking for lazy people.
The color comes from the amount of oil the plug is exposed to. The richer the mixture, the more oil (assuming the same fuel/oil ratio). Zero oil will produce zero brown coloration. Gasoline itself leaves a little deposits but not of a brown color. So if the difference between a right and wrong main jet is 8% then that means you 1) need to visually be able to tell the difference between that % of color change, 2) need a point of reference, a color that is correct for all two stroke engines that ignores any need for extra fuel to cool the piston due to excess RPM or compression. But neither of the two is correct.
How to jet - You have to try different jets in a normal high load situation (uphill, sand, mud, etc) to see which one runs clean and gives the best power. If two jets seem the same then choose the bigger of the two for reliability.
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Discussion Starter · #2 ·
And looking at the ceramic ring is just as useless because in this case the rings would all be the same. Rings change as the plug color changes and in this case the color didn't really change. So the porcelain ring would of been the same too. Proof of that assumption came from someone who shared the pics below of plugs with the threads cut off to see all the porcelain. The widths of the dark bands were all the same. Here's my web page on the subject: Jetting Without Plug Reading
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