Topic: Plug readings

[John]

hey guys,
here's my latest problem. I worked on the carbs all weekend. After a 20 minute test ride the plugs looked very clean with some white deposits on the ground-strap electrode. That should mean i'm running lean. The issue i have is that 1. my colortune says the mixture is adjusted correctly. 2. it runs f-ing great. no popping or racing. So i have 3 questions for you:

1. Is it possible that in switching my fuel this weekend from 87 to 93 i bought fuel with some mystery additive that is causing this plug color?

2. Will i get more symptoms than just plug color if i am so lean that i am doing engine damage.

3. if you look around the web, people don't agree as to where on the plug they are pulling their readings -some, the tip of the electrode, some, cutting the treads and reading the base of the electrode, others, the porcelain insulator, the base ring of the plug, or finally, the ground-strap electrode. What part do you use to judge color.

thanks,
john

[eldar]

Your mix may be ok but higher octane burns hotter and is probably what is causing the white tips. You may want to richen the mix just slightly. I say go back to the 87 octane. The 93 does not offer much of an increase in performance and is not going to be worth the cost.

[Terry in Australia]

Far Canal! 87 Octane, where do you live, Africa? Geez, in Australia, the lowest unleaded fuel is 91 RON, and our Hi Octane stuff is 98! In the good old days before lead was banned and replaced with more dangerous chemicals, (well how many people outside of Iraq do you know who died of lead poisoning?:-) you'd look at the porcelaine insulator and if it was a nice tan color, your mixture was pretty much spot on, but as unleaded fuel tends to burn black anyway, a light grey is about as good as you'll get. I'm with Eldy on this one, you might be running just a tad lean, so try raising your needles one notch, and see what that feels like, if anything, it should feel better. Cheers, Terry.

[MikeDeB]

Your mix may be ok but higher octane burns hotter and is probably what is causing the white tips. You may want to richen the mix just slightly. I say go back to the 87 octane. The 93 does not offer much of an increase in performance and is not going to be worth the cost.

Actually, the amount that higher octane fuel burns hotter as compared to lower octane fuel is negligeble.  What will happen from using high octane fuel in an engine designed for lower octane fuel will be a greater amount of carbon deposits on the plugs, valves, piston heads and exhaust port.  Which will lead to fouled plugs.  This is because high octane fuel has more anit-knock compounds in it to slow the burn down and prevent detonation in high compression engines.

[TwoTired]

1. Is it possible that in switching my fuel this weekend from 87 to 93 i bought fuel with some mystery additive that is causing this plug color?

Yes, quite.
Higher octane gasoline does not make more power in these old non-computer controlled engines.  You only need enough octane to prevent detonation and preignition while the spark is advanced to make peak power.   Higher compression ratios require a higher octane and/or retarded spark to prevent preignition with marginal octane ratings.  Today's higher octane fuels are blended with extra additives that will leave deposits in engines that don't have the compression to require these fuels.  It is possible that your white electrode deposits are artifacts of the fuel you are using.
 Octane ratings measure a gasoline's ability to resist engine knock, a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders.  It is not a rating of energy content.  Most gas stations in the US offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane) and premium (usually 92 or 93) depending on the state where purchased.
See:  http://www.repairfaq.org/filipg/AUTO/F_Gasoline6.html  For more on fuel ratings.

2. Will i get more symptoms than just plug color if i am so lean that i am doing engine damage.

Absolutely, detonation is quite audible as is pinging under load. Check out:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
for plug indications of pre-ignition and lean mixtures.  This was produced by NGK who knows quite a bit about spark plugs and how to read them for mixtures and proper heat ranges.

3. if you look around the web, people don't agree as to where on the plug they are pulling their readings -some, the tip of the electrode, some, cutting the treads and reading the base of the electrode, others, the porcelain insulator, the base ring of the plug, or finally, the ground-strap electrode. What part do you use to judge color.

The mixture and heat range of the plug are interactive factors.  If you look at a cross section of a spark plug, you'll note that the length of the insulator varies with the heat range.  This is where heat is carried away.
http://www.ngksparkplugs.com/techinfo/spark_plugs/overviewp2.asp?nav=31000&country=US

 Reading the plugs for mixture and heat range is done at the insulator because the heat for mixture and heat range should be high enough to burn away deposits so the plug will be self cleaning.

If your plugs aren't showing signs of preignition, you can't hear any knocking or pinging under load, and the plug insulato, particularly near the tip, is an agreeable color, ride on and be happy.  It's up to you as to whether you want to waste your money on higher octane rated fuel than you actually need.  If your CR is still less than 9.5 to 1, US 87 octane is all you should need.

Cheers,