Reading Spark Plug: Black 'Soot' Color

pgtrSeptember 21, 2008

I did a little basic (near) end of season maintenance to our old ~1986 Murray 20" push mower with B&S 3.5HP (92502). Pulled the plug to replace since it's been about 5-ish years since it was installed.

The old plug had a flat black sooty coating on it. There was actually minimal 'wear' to the plug as far as the tip, gap or gnd electrode.

Q: What if anything should I look for to better optimize engine performance/life?


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New air filter.

    Bookmark   September 21, 2008 at 10:18PM
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Plugs are read after a long, hot run at full power, with the engine being immediately shut off. Any other way may give any result you can think of. Assuming it was done correctly, the obvious answer is a rich condition. A clogged air filter could certainly cause it.

    Bookmark   September 22, 2008 at 9:24AM
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It depends on how thick the coating is. In my learning, a properly running engine WILL deposit a thin "sooty" coating on the plug. It's just what happens through combustion. So it sounds normal to me. What you really want to be looking out for is soot that's so thick you can knock off chunks of it at a time, oil, or a glazing on the plug. Oh yeah and making sure the ceramic and electrode are intact and havn't been damaged by detonation or anything. I agree though, keeping your air filter clean and using clean fresh fuel are the best ways to maintain performance. Speaking of air filters - they're easily cleaned out; both paper and foam elements. Clean out a foam element with gas by soaking and squeezing the debris and dirt out of it. Don't forget to resoak it with oil(if required) before reinstalling it. As for paper elements, you can usually knock them out and use a little low pressure compressed air to get them clean. If you can hold it up to a light and see it through the filter I'd say you're good to go. Just make sure the gasket surround isn't cracked or broken down and that, obviously, there aren't any tears or holes.

    Bookmark   September 22, 2008 at 11:23AM
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A properly running engine will not deposit a thin sooty film on the plug. A thin sooty film on the plug indicates an overly rich condition however minor it may be. You might think it's running right but it could be tuned to burn more efficiently and the plug would then be tan in color if everything is right. Cleaning paper filters is not recommended. Blowing them out with air can damage the paper. One small hole and you will have a worn out engine under even moderate conditions. Remember, only that thin piece of porous paper stands between your engine and the dirty grime in the air. Unfiltered dirty air will wear that engine out in no time at all. Replace air filters that you suspect to be dirty. Don't jar them around or blow air through them. That's asking for trouble.

    Bookmark   September 24, 2008 at 1:13AM
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I beg to differ. If you were to show me a perfectly clean plug and said it came out of a strong running engine with more than 20 hours on it and claimed it was due to superior tuning I'd think you were a whack-job. Unless you're running it so lean that you're losing potential power, there's no way. A light coating of soot on the plug is normal on an engine with a balanced fuel to power ratio. AND, that light coating does nothing to affect the performance of the plug. As for cleaning a paper air filter, yes, it can be done with proper care. I, and many others I know, have used this method for years upon years with great success. All it takes is a careful eye to inspect for anything potentially damaging. I wasn't talking about using 100 or 80 or even 60psi. That's why I said low pressure, sorry I guess I should have been more specific, but any bone head should know that paper is delicate. And just as an observation, a hole, say one the size of a pin hole, even a big pin about 0.030" across, would only have an area of about 0.0007sq". On a typical lawnmower air filter of 3x4=12sq" that's only 0.006% of the total surface area IF it was a flat piece of paper. But then it's *not* a flat piece *is* it. ...Jeez-you've got me ranting. Sorry about that. Look, there's nothing wrong with carefully cleaning a paper filter. Replacing it every time it's dirty supports that obsessive jiffy-lube mentality.

    Bookmark   September 24, 2008 at 8:50AM
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I have cleaned paper filters too but it's not really good practice in my opinion. I replace them now. I may be a whack job but tan plug color is indicative of a properly tuned engine. Period. Soot on the plug is a rich condition. Period. All the manufacturers service manuals and spark plug mfgs. information totally backs that up. Check the plug color chart on the link. Plug number 14 is from a perfectly tuned engine.

Here is a link that might be useful: plug color chart

    Bookmark   September 24, 2008 at 12:31PM
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Then we've actually been arguing the same point. That powdery brown coating is what I was calling the soot. I was under the impression that you were saying that the plug would be clean. Anyway, let's get off of this for now. I think the link you posted should be able to clear it up for anyone.

    Bookmark   September 24, 2008 at 1:36PM
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There ya go. A simple misunderstanding.

    Bookmark   September 24, 2008 at 3:37PM
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If this mower has the B&S "L" head engine on it, well this engine design is not known for perfect combustion. The plug nose will vary from grey to black in colour. The Intec engine on mine is OHV design and the plug nose is tan in colour. It is a much more efficient combustion chamber design with a cleaner burn.

    Bookmark   April 22, 2010 at 5:35PM
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Kersplat , Wow you really went back in Time for this Rebuttal Dude lol !

P.S. Ray : Good Plug Colouration and Indexing Chart , it also goes a way back , Motorcycle Institute 101 ! :)

    Bookmark   April 22, 2010 at 6:05PM
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