Steve:
Combustion doesn't take place around the plug body (only between the electrodes) so I'd expect to see the bottom of the threaded portion slightly blackened in comparison to the electrodes.
.
Clarence
Kiss my Keyster (carb kit needles)
Steve: combustion starts between the electrodes but it burns thru the whole Combustion Chamber, if your getting good combustion.
Clarence
I should've clarified: 'Ignition' rather than 'combustion'. As in the bang that creates the combustion and, hence, the expansion that drives the piston back down the cylinder. My mistake.
Steve:
What, in the name of Newtons Laws of Physique's, are you going on about here! Are you saying that the body isn't necessary?, for the engine to run, what about all them hot gases poring out of the spark plug hole without the Body to keep all them hot gases where they belong; instead of up your...pant leg. [quote="e3steve" But, I have to say, I've never seen the plug body anything other than considerably darker than the electrodes -- Bosch surface-discharge plugs notwithstanding. The logical part of my grey matter tells me that it's because of the mass difference, whereby the body doesn't reach the temp of the electrodes and insulator and, therefore, takes no part in the creating of the spark or combustion process.[/quote] Cheers Clarence :-)
Clarence, far be it from me to challenge any of Newton's Laws of Physics. If you're serious, or being just plain argumentative, please read, and comprehend, my posts again.
If you view the 'Common spark plug conditions' chart document (by clicking on it to open, then click again on the opened image to size it up) that I posted for you (in order to help), you'll see that none has anything other than a darkened body; and that is in spite of, and irrespective of, the colour and state of the electrodes or insulators. Not that it couldn't occur -- just that, in over forty years of playing with petrol engines, I've never seen such a phenomenon. Still, as I've always stated: 'every day's a schoolday!'. As always, I stand to be corrected. If you're not serious, nor being just plain argumentative, please ignore the foregoing.....☺ Let me stick my nose in here. The combustion process produces by products and that is what you are looking for on the plug. The portion of the plug that obtains the highest temp will give you a good idea of how hot the combustion process is. So the electrode, insulator and the ground electrode are all hanging out in the middle of the combustion flame and getting hotter than any other portion of the plug. They are also exposed to the dynamics of the combustion process, which is pretty nasty. The body is transfering heat to the head so it will always be cooler.
If the flame is lean, it produces little to no carbon. There is also much less fuel to help cool the parts when it first rushes in. I also believe, but am not sure about this, that the radiant heat is higher in a lean flame because there is less "stuff" for the heat to radiate thru. By stuff I mean products of combustion. So those bits of a plug that are out in the chamber, with very little to transfer heat to, get hotter. That's why a very lean mixture will start to cook the plug tip. The metal is approaching its melting point because it cannot get rid of the heat fast enough. A rich mixture produces carbon because of incomplete combustion. The carbon deposits itself on the plug out at the tip, and pretty much everywhere else. But because the tip is hanging out there in what you might call extremely violent conditions, the carbon will not build up equally. Down around the inside of the base, there is much less turbulence and probably no flame front since there is little to no flow in that area, so the carbon builds up and never really gets blown off. But out at the tip, the carbon builds up and is blown off by the violent combustion process. If the engine is rich at idle, which they usually are, it will deposit carbon on the tip and then at speed when the mixture leans out, the carbon will be blown off. This is why the chop throttle method is key to properly reading plugs. If you go for a WFO ride and then idle as you putt down the driveway, your plug is going to read the idle mixture coming down the driveway. Getting back to rich mixtures, only in extremely rich conditions is so much carbon produced that it builds up to the point of bridging the electrodes and shorting out the plug. Of course I'm not talking about bikes that burn oil. That's an entirely different issue. This brings up heat range of a plug. A hotter plug is designed to run at a higher temp than a cold plug The range is selected both to prevent pre-ignition (too hot) and deposits) too cold). Two stroke engines tend to run hotter plugs because of the oil in the fuel. The plug range is determined by the depth and thickness of the porcelain insulator for the center electrode. . The heat range property of a plug makes me want to think that carbon can be burned off, but I doubt that. I think the heat range is more focused on burning off deposits made when the fuel mixture first enters the chamber and cooks onto the surface of the plug. It is that, which the heat range feature burns off... I think. So, to eSteve's point, reading the base color is of little value because it is not out in the storm, like the tip of the plug is. It probably starts to get black with the first few firings and reaches a point where it just stays that way, regardless of the mixture. But the tip of the plug is constantly changing with the mixture. Probably the only thing that would not change is damage caused by overheating the plug with a severly lean mixture. So why does a perfect mixture produce a tan plug tip. I really don't know for sure. I suspect it is a combination of some products of combustion, but more just the color the plug tip becomes when exposed to the combustion process at the correct temperature. I've been told by mechanics that work on cars today that reading plugs is not very useful because the mixtures are much leaner with FI engines and the additives in todays fuels make different deposits than they did back in the day of carbs. I have no empirical proof of that. I suspect there are probably articles on the web about it, if you look. Want to get to the bottom of the mystery. Take your bike to a shop with a dyno and exhaust gas analyzer. Run throttle chop tests across the different fuel circuit ranges of your carbs and compare the plug readings with the exhaust gas analyzer readings. That will tell you what a plug should look like if the mixture is correct. I saw an exhaust gas analyzer that is small and could be strapped to a bike for road testing. I thought about buying it just to satisfy my curiosity on jetting carbs. With dual exhausts you could do each carb separately. Everyone says the left cylinder on old Triumphs always runs leaner than the right. I could finally put that legend to bed. But, alas, I never did buy the thing. Don't even remember where I saw it anymore. New Info : After initially posting this, I did a quick search and found this NGK site. It's a great tutorial on spark plugs. So I've edited the post to include the link. This should clear up any questions. http://www.ngksparkplugs.com/tech_suppo ... p?mode=nml regards, Rob
Rob, a well-composed post! And a great tutorial with the NGK link.
None of it is rocket salad, though; and all stuff I (I'm sure 'we') learned in my (our) biking & motoring teens. I could 'read' my Cooper & Cooper Ss -- I ploughed through 3 Minis before moving on -- and tune the twin SU carbs with my eyes shut (but my ears open). Static timing-up & reading the plugs' colours gave all the info that was needed for mixture; optimum balancing was always attained using a length of rubber tubing and listening to the induction rush at the carbs' throats. I apply the same techniques to my bikes; they still work nowadays. And I'm still only really interested in the plugs' colours. An over-abundance of oxygen, mucker. More oxygen = hotter burn. Look at how an oxyacetylene cutting torch behaves: it doesn't do much damage to the workpiece until the oxy-lever is applied, at which point it annihilates in the blink of an eye. Interesting post...like any other subject, the more you dig the deeper it gets!
There are varying theories regarding which part of the plug tells you what, and the plug manufacturer's pics show different extremes which should be obvious to correlate with in general. The fine tuning seems to be the hard part. Some tuners even go to the extreme of cutting off the threaded portion of the plug to read the "mixture ring" at the base of the insulator hidden in the well of the plug, and claim that is the true indicator of mixture ratio. Some look at the length of the discoloration of the grounding arm of the plug. Some say you need a 10X magnifying glass to read one. I always used the tip of the electrode and the end of the grounding arm as the critical areas since they should get the hottest, ........and the formula "black is bad, white is worse, and tan is just right"....but maybe I need to get a 10X glass and dig deeper? Further reading: http://www.nightrider.com/biketech/spkplghnbook.htm http://www.ngksparkplugs.com/tech_suppo ... aqread.asp http://faq.f650.com/FAQs/SparkPlugFAQ.htm#Plug Chop http://www.kawieriders.com/forum/kawasa ... -lean.html With a little work we could turn this into an oil thread. 66 dream, 78 cb750k, 02fz1, 09 wing
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