Reading your sparkplug

It is necessary to know all these different plug configurations if you are to be completely successful in doing your own maintenance work, and it is absolutely essential that you know how to "read" plugs if you're dealing with a high-performance bike (whether factory-built or do-it-yourself). Sports/touring machines usually are well sorted out before they're sent to market, but even the best racing bikes seem to be timed and jetted a little off-the-mark for our fuels and riding conditions. We suspect that the laboratory-quality gasoline that some factories use in their development work warps manufacturers' ignition advance recommendations; whatever the cause, nearly all the factory-built racing engines with which we have direct experience run better when their spark timings are slightly retarded. Typically, too, their spark plugs are one heat range too cold and they're jetted a bit rich. Also typically, these same bikes are fitted with even colder plugs, richer jetting and sometimes are given more spark advance by those who buy them.

The worst, most destructive, combination of mistakes we see begin with two widely-held assumptions: first, that a cold spark plug will help fend off that old devil detonation; second, that more spark advance -not less- is the thing to try when reaching for power. Try to use a too-cold spark plug and you very likely will have to jet for a lean mixture to avoid plug fouling - and as you lean an engine's air/fuel mixture down near the roughly-14.5:1 chemically-correct level it becomes extremely detonation-prone. Excessive spark advance is even worse in its ability to produce detonation, and when combined with a lean mixture it's enough to quickly destroy an engine.

Most people who've had some experience with racing bikes (especially those with two-stroke engines) know that detonation is a piston-killer. Few really know the phenomenon for what it is: a too-sudden ending to the normal combustion process. You may imagine that the ignition spark causes an engine's mixture to explode, but it actually burns. There's a small bubble of flame formed at the spark gap when ignition occurs, and this bubble expands - its surface made a bit ragged by combustion chamber turbulence - until all the mixture is burning. This process begins slowly, but quickly gathers speed because the mixture beyond the flame_ bubble is being heated by compression and radiation to temperatures ever nearer the fuel's ignition point. When the initial spark is correctly timed the spreading flame bubble will have almost completely filled the combustion chamber as the piston reaches top center, and all burning will have been completed by the time the piston has moved just a millimeter or two into the power stroke. But the final phase of this process can be shifted from simple burning into a violent detonation of the last fraction of the whole mixture charge.

Starting the fire too early will produce detonation, as it gives the mixture out in the chamber's far corners time enough to reach explosion-level temperature. And a slightly lean mixture detonates at a lower temperature. It's all a function of ignition timing and mixture in any given engine, and spark plug heat range plays absolutely no part in it.

Your engine's spark plug doesn't cause detonation but it can tell you when and why the phenomenon has occurred. Moreover, the spark plug can tell you with remarkable precision how much spark advance and what jetting your engine needs. Those are things you can "read" in a spark plug, and all that is written there will be revealed very clearly when the heat range is right.

So how can you tell whether you've chosen the right heat range? It's easy: a spark plug should be getting hot enough to keep its insulator nose completely clean, with all deposits burned away, but not so hot that its electrodes show signs of serious overheating. These are things to look for on a new plug that has been subjected to a few minutes of hard running. After many miles of service insulators acquire a coating of fuel deposits, with some coloration from oil in two-stroke applications, and there will be some erosion of the electrodes even when everything is normal. Don't try to read old spark plugs; even the experts find that difficult. New plugs present unmuddled information about what's happening inside an engine, and can give you a complete picture after just minutes of hard running. At least they will if they're running hot enough, and that should be hot enough to keep the insulator clean.

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 a magnifying glass 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.

Once you have brought your engine's ignition timing close to optimum you'll almost certainly have to make a further change in spark plug heat range. Manufacturers' specifications for racing models very often advise you to use too much advance and a too-cold plug, and when you shorten the spark lead to suit commonly-available fuels it almost certainly will be necessary to use a warmer plug. Then, when you have found plugs of a heat range that will keep that insulator nice and clean you can start adjusting your engine's air/fuel mixture - a task that will be easy if you can forget everything you thought you knew about this aspect of plug reading.

A lot of amateur tuners, some of whom are fairly successful, will look at some plug freshly removed from a two-stroke engine and offer advice based on the color of the oil deposited on 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 two-stroke engines often will respond favorably to a slightly richer mixture, which provides a measure of internal cooling; some four-stroke 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. As previously noted, the 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. 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.

We cannot stress too strongly the need to give spark advance your closest attention, because excessive spark lead is the most frequent cause of detonation, which is a real engine killer. You can't stop advance-produced detonation with a cold spark plug, nor with anything but a wildly over-rich mixture. Also, excessive ignition advance has a bad effect on performance. We ran a 250cc road racer at the drags a few months ago, and found that retarding the spark about five degrees from the manufacturer's setting raised the trap speed from 106 to 110 mph. Similarly, there's a 125cc motocross machine residing in our shop which runs a lot stronger and cleaner since it has been retimed for less advance, jetted leaner, and been given a hotter spark plug.

Even touring bikes sometimes benefit from revised spark timings. Only rarely will their carburetion be off enough to need attention, but the ignition advance they get represents a compromise between the optima for power and economy. For some riders, especially those who use a lot of throttle much of the time, stock ignition advance is too much advance. And of course many riders find that their specific requirements are better met with non-standard plug configurations.

The trick in all this is to know enough about spark plugs to be able to choose the right basic type, and to understand what the plug has to say about conditions inside your bike's engine. It's not an altogether easy trick to perform, with so many things to be remembered all at once; it's a terrifically effective trick when you get it right.


http://www.strappe.com/plugs.html



Frequent question: So the carbon band thickness is a result of all of the jets? I thought you said that this was how you determine the condition of the main jet.


It is how you read the mainjet, but if you are riding around at 1/3 throttle opening with a needle that is too rich and the engine never sees WOT then the reading you get will only be for the circuit it's running on. Carbon collects when the combustion is incomplete regardless of the circuit. The ring is used to read the mainjet because WOT is the only time you can be sure you are getting the same reading (i.e. throttle opening) on the plug. Given the tapers of needles there is no consistent way to read 1/4 1/2 or 3/4 throttle on the insulator base.

Here is a picture of a proper jetted main jet with maybe one heat range too cold of a sparkplug:
Mtel base shell have ever term you use has been removed.