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i found this a good read
The following are some of the things that will give you some input on plug reading .Keep in mind that most of the info will be valid only if you do a plug chop or, if on the dyno, when doing a full load test or steady state test. Also, plugs are best read with a 10 -15x illuminated viewer. The absence of lead in fuel today will make plug reading more difficult if not impossible as compared to the older leaded fuels where you could tune by the color of the plugs.
A lot has been written about how to read plugs, I've boiled it down to three areas of the plug:
The ground strap and center electrode ... shows heat range and timing.
The base ring and the junction of the porcelain to the plug shell ... shows AFR/jetting
The porcelain ... signs of detonation/pre-ignition
Pre ignition/Detonation:
The porcelain shows up pre ignition/detonation, it will not accurately reveal jetting/air/fuel ratios. To look for the first/beginning signs of detonation, search the white porcelain for tiny black specks or shiny specks of aluminum that have fused to the porcelain. When detonation occurs, part of the air/fuel mixture explodes instead of burning, the explosion is heard as a "metallic knock", this audible knock is the result of a sound shock-wave, this shock wave travels back and forth across the clearance volume "disrupting" the cooler boundary layer gases that cover the entire clearance volume area. This disruption allows "more" heat to be transferred into parts, especially, domes/piston tops,...along with the very rapid rise in pressure like a hammer blow, pistons can get torched with melted sides and holes. With the early signs of detonation, the shock-wave will also rattle rings causing the tiny amounts of oil that now gets by the rings, to be fused to the white porcelain as tiny black specks, also fused as specks are soot that was clinging to clearance volume surfaces in the relatively "still-air" of the boundary layer. One step beyond the black specks, will be tiny specks/balls of aluminum coming off the pistons that will be fused to white porcelain ...the next step to be reached is occasional pieces of the porcelain being broken-off as detonation gets worse. Soon after that are holes, blown head gaskets, broken connecting rods, and so on. It should be noted that in some cases the early signs of detonation may be present without being audibly detected. Also, some black specs may show up on the insulators from oil passing by intake valve guides or seals and oil entering the chambers from crankcase ventilation "re-breathers". These specks will usually be on the portion of the insulator that faces the intake stream as opposed to the entire insulator.
Additional signs of beginning detonation are piston rings. By comparing a ring's "free-diameter" to the" out-of-box" free-diameters of new unused rings it can reveal beginnings of detonation before much harm is caused. This is a result of the above previous explanation about heat being transferred more readily because the of the insulating/cooler boundary layer being disrupted by the shock-wave. A piston ring is a simple spring, when a spring is over-heated it will loose tension.
The very tiny sharply defined porcelain -"ditch" that encircles the center electrode, will also show up early signs of pre ignition/detonation and the wrong heat-range. Look for signs of the ditch beginning to be filled up with melted porcelain, you will need a 10x - 15x illuminated viewer.
Heat Range/Timing:
The ground strap indicates the heat-range of the spark plug. If the "color" of the ground strap "changes" too close to the ground strap's end, (which is above the center electrode), then the heat-range is "too cold" , meaning that the strap is loosing heat too quickly to the base ring, and is not able to burn off deposits until near its end. If the "color" of the strap changes near where it is welded/attached to the base ring (last thread ring), then it means that the plug heat-range is "too hot", because heat is not being transferred /cooled from the strap to the base ring quickly enough. The strap might begin to act like a "glow-plug", eventually causing pre ignition and/or detonation later on. Proper heat-range is when the "color" is at the half-way point on the strap, neither too cold or too hot.
Color ... meaning the evidence of heat/or lack of heat by the appearance dark vs lightened color of metal
Some look for a blue line on the strap as an indicator of proper advance but I have found that to be a poor indicator of timing, the center electrode can tell you more about advance.
It's impossible to separate the question of ignition advance from the primary evidence of spark plug overheating, which is most strongly shown on the plug's center electrode. If you inspect this electrode's tip with an illuminated viewer and see that its edges are being rounded by erosion, or melting, then you know there's overheating. You should also have a close look at the tip of the ground electrode, checking for the same symptoms. Finally, inspect the condition of the insulator, which should be white but with a surface texture about like it was when new; a porous, grainy appearance is evidence of overheating. If the signs of overheating are confined mostly to the center electrode you can bet you're using too much ignition advance. Retard the spark timing in small (two or three degrees) increments and as you get close to the optimum advance you'll find two things happening: first, the whole plug will be running colder; second, the center electrode will begin to acquire a film of fuel deposits extending out from the insulator nose toward its tip.
The fuel film mentioned here is what you watch when making fine adjustments in ignition advance. In an engine that's been given just a few degrees excessive advance (as most have) the fuel film will only extend outward along part of the center electrode's exposed length, ending abruptly a couple of millimeters from the tip. The portion remaining won't be filmed over simply because it has been hot enough to burn away the fuel salts dusted on the rest of the electrode, and you'll see that sort of localized overheating created by too much spark advance even on a plug that is two or three heat ranges too cold. And you'll have the correct spark advance when the center electrode's fuel film continues right out to within a hair of its tip. There are a couple of caveats to be observed in this matter. An overly-retarded spark timing won't show except as an absence of any evidence pointing to too much advance. Also, the spark itself will blast clean spots in the electrode's fuel film, and when there's enough combustion chamber turbulence to blow the spark sideways into a curved path you'll get a cleared area on one side of the electrode. This lop-sided spark blush shouldn't be mistaken for the more sharply defined ring associated with the electrode tip overheating produced by excessive spark advance.
AFR/Jetting ... The base ring and porcelain/plug shell junction:
A lot of well meaning people will look at some plug freshly removed from an engine and offer advice based on the color of the insulator nose. In fact, if the plug is hot enough there won't be any color, and if there is that still has nothing much to do with air/fuel mixture. If you think about it you'll realize that the only color you can get from an air/fuel mixture is the color of soot. When the mixture trapped in an engine's combustion chamber has more fuel than can be burned with the available air, then combustion will be incomplete and the excess fuel will remain as soot, which is not brown or tan or magenta or any color other than black. And if your engine's mixture is too rich, the sooty evidence will be present on the spark plug's insulator, in a very particular area.
You won't find any soot out near the insulator nose, on a plug that's running hot enough to keep itself from fouling, because temperatures there are too high to let soot collect. But the insulator is much cooler deep inside the plug body, and coolest where it contacts the metal shell, which is precisely where you "read" mixture strength. Look far inside a plug, where its insulator joins its shell, and what you'll see there if your engine's mixture is too rich is a ring of soot. If this ring continues outward along the insulator to a width of even a millimeter you can be sure the mixture is rich enough to be safe, and too rich for maximum output. In most engines best performance is achieved when the mixture contains only enough excess fuel to make just a wisp of a "mixture ring" on the plug insulator. Air cooled engines often will respond favorably to a slightly richer mixture, which provides a measure of internal cooling; some engines give their best power when the mixture is leaned down to such extent that the last trace of soot deep inside the plug completely disappears.
Never try to jet too close to a best-power mixture until after you've taken care of spark advance. In general, an air/fuel ratio that yields maximum power is only a shade richer than the one that is most detonation-prone. Fortunately, the plug will tell you when there has been even slight detonation inside your engine. As previously mentioned , the signs to look for are pepper-like black specks on the insulator nose, and tiny balls of aluminum concentrated mostly around the center electrode's tip. Severe detonation will blast a lot of aluminum off the piston crown, and give the plug a gray coating-which is a portent of death for the engine. A few engines will show just a trace of detonation when jetted and sparked for maximum power, but that never produces anything more than a few miniscule spots of aluminum gathered on the center electrode's sharp edges. If you see more aluminum and an extensive peppering evident on your plug, you're in trouble.
The classic "fuel ring" method of reading mixture outlined above can be hard for most to read accurately, and, with some plugs can be difficult to see even with a good illuminated viewer. All is not lost, there is another method that works just as well. I've found this method to correlate to readings from an AFR meter as well as the classic method.
The air/fuel mixture ratio shows up on the base ring (the last thread ring, it has the strap welded to it). You want a full turn of light soot color on the base ring. If you want to tune for maximum power, then you want 3/4 to 7/8 ths of a full turn of light soot color to show up on the base ring, this is on ragged-edge of being too lean, but will make the most HP on most engines. To be safe, leave it at a full turn of light soot color. If the base ring has a full turn of color, but there are "spots" of heavy build-up of "dry soot" on top of color, then jetting is too rich . Even if the porcelain is bone white, the jetting is still too rich. Do not look at the porcelain to read jetting .
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O.K., I guess we could call this plug reading 102! In plug reading 101 there was a caveat that the readings would only be valid under certain conditions, the exact wording was: "most of the info will be valid only if you do a plug chop or, if on the dyno, when doing a full load acceleration or steady state test" So, plug reading 102's purpose will be a clarification of the caveat. To read a plug, you must first prepare it properly for reading, so it will give you the real story. That means you can’t just go over to an engine and yank the plug. You have to know how the machine was operated in the period just before you look at the plug, and the machine must be run in some steady condition for a while before taking the reading. You can’t just make a fast pass down a road, then pull into your garage and let the engine idle for a while before shutting it off. This kind of a reading from a spark plug will be totally worthless. A plug can tell you only about a recent, sustained, single operating condition. First off, the engine should be in reasonably good tune and mechanical condition, and the plug should also be in reasonable condition and the plug proper for the engine. New plugs will give the best readings, but a plug that has not been subjected to grossly out of range operating conditions will work also. To check for main jet sizing or WOT mapping for EFI, you need to do a full throttle run. This is best done on a Dyno, mostly due to safety. However, it can be done on a deserted section of road. A road with a slight uphill incline is actually prefered. A WOT run thru the gears including top gear is best, and requires at least a half mile. If you try this with a worn engine, or one in bad tuning condition, it may not last! This is one time where the 5th gear WOT dyno run has merit ... not because of the possible higher numbers, but because it more closely reflects the WOT jetting/mapping the engine will see under high load WOT conditions on the road. I like to do this plug read on the dyno by going thru all the gears because it keeps the engine under WOT conditions longer and gives a better read. Yes, I know that AFR meters are supposed to eliminate all of this, but I'm a little "old school" and I don't tune strictly by AFR, CHT, and EGT alone. Even the results obtained from a "good" WOT run on a dyno may not match up with results obtained from road data. My use of on board data acquisition equipment has shown this to be true. It's usually close, and certainly better than nothing. To me, the best final tune data is that derived from actual road/track data. Anyway, assuming that the engine survives , at the end of the run, pull in the clutch and kill the ignition. Coast to a stop or brake the dyno and don’t run the engine anymore until you yank the plug. That is called cutting clean. What the plug will now tell you is how things were inside the engine at full throttle. Applying what was covered in plug reading 101 is the next step. The next step is a second plug reading, and this one should be taken at mid-throttle or cruise conditions. The full-throttle test tells you about the condition in the combustion chamber while it was on the main jet, because that’s what controls mixture strength at full throttle. For EFI this test is checking the fuel maps in the part throttle/light MAP load areas. The half-throttle or cruise reading tells you about the needle and needle jet (or mid-range jets) , because they control that part of the engine performance. Most average tuners won’t even perform this part of plug reading and tuning, and will be happy with full throttle runs and readings. The procedure is pretty much the same. Get the vehicle up to the cruise condition you want to check for, sustain that condition long enough to ensure good plug coloration ( 1/2 to 1 mile), the do the chop, coast, and read routine. You can do multiple reads in this range to see if there are any really bad areas ... too lean or too rich. Plug reads done in this manner can be invaluable when chasing either fuel economy or pinging issues. The above part throttle type tests are what is done when doing fuel mapping on a dyno using the load cell. You can't do this very well on an inertia type dyno ... yes, it *can* be done by a very talented tuner/operator, but most don't even bother. Absorption type dynos lend themselves to this type of testing very well, they work best for step or steady state testing. Inertia type dynos work best for acceleration/WOT type testing ... IMO.
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an interesting read... 
I read this the other week myself ..... 
, knock the engine off and free wheel into the layby, pull the plugs out. 
