Spark Plugs, Iginition Timing, Carb Jetting.
Smart people who have had some experience with Racing Bikes know that detonation destroy pistons. Some know about the phenomenon, to sudden of a ending to the normal combustion process. As previously mentioned an ignition spark doesn't cause an engine's fuel mixture to explode, it actually burns. A small kernel of flame is formed at the spark gap when ignition occurs, this kernel expands, its surface is 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 kernel is being heated by compression and radiation to temperatures nearer the fuel's ignition point. When the initial spark is correctly timed the spreading flame kernel will have almost completely filled the combustion chamber as the piston reaches top center and all the burning will have been completed by the time the piston has moved just a millimeter or two into the power stroke.
The last phase of this process can change from simple burning into a violent detonation of the last of the mixture charge. Starting the fire too early will produce detonation as it gives the mixture that is out in the combustion chamber's far corners time enough to reach explosion level temperature. A slightly lean mixture detonates at a lower temperature. It's all a function of ignition timing and mixture in any given engine, and a spark plug heat range plays absolutely NO part in it.
A spark plug doesn't cause detonation but it can tell you when and why the phenomenon has occurred. A spark plug can tell you with remarkable precision how much spark advance and what jetting your engine needs. These are signs that you can read in a spark plug, and are there when the heat range is right.
How can you tell whether you've chosen the right heat range? 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 the signs to look for on a new plug that has been subjected to a few minutes of hard running. After many miles of service insulators will get a coating of fuel deposits and there will be some erosion of the electrodes even when everything is normal. DO NOT try to read old spark plugs even tunning experts find that difficult.
New plugs present clear information of what's happening inside an engine, and can give you a complete picture after just minutes of hard running as long they were hot enough 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 located mostly on the center electrode you are using too much ignition advance. Retard the spark timing in small two degree increments and as you get close to the optimum advance you'll find two things happening, first the whole plug will be running colder and 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 is what you watch when making fine adjustments in ignition advance. In an engine that's been given just a few degrees excessive advance 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 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 heat ranges too cold.
You will 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 warnings to be looked concenring this matter. An overly retarded spark timing won't show except as an lack of any evidence pointing to too much advance, and the spark itself will blast clean spots in the electrode's fuel film when there's enough combustion chamber turbulence to blow the spark sideways into a curved path, you will get a cleared area on one side of the electrode. This lop sided spark marking should NOT be mistaken for the more sharply defined ring associated with the electrode tip overheating produced by excessive spark advance.
Once you have set your engine's ignition timing close to optimum you'll almost certainly have to make a further change in spark plug heat range. When you have found plugs of a heat range that will keep that insulator nice and clean you can now start adjusting your engine's air/fuel mixture a job that will be easy as long as you can forget everything you thought you knew about this aspect of plug reading.
If the plug is hot enough there won't be any color, and if there is that it still has nothing much to do with air/fuel mixture. 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 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.
Soot won't be found near the insulator nose on a plug that's running hot enough to keep itself from fouling, because temperatures located there are too high to allow the soot collect. 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 if your engine's mixture is too rich you will see 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, yet 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. Suzuki 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 jet too close to a best power mixture until after you've taken care of spark advance. As previously mentioned, the air fuel ratio that yields maximum power is only a shade richer than the one that is most detonation prone. The spark plug will tell you when there has been even slight detonation inside your engine.
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 sign of death for the engine. Some 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.
I cannot stress strongly enough the need to give spark advance your closest attention, because excessive spark lead is the most frequent cause of detonation, which is an engine killer. You can't stop advance produced detonation with a cold spark plug, nor with anything but a very over-rich mixture.
Excessive ignition advance has a negative effect on performance. Engines runs a lot stronger that are set up with less advance, jetted leaner, and been given a hotter spark plug.
Street bikes sometimes benefit from revised spark timings. It is very rare that their carburetion be off enough to need attention, but the ignition advance they have is a compromise between the optimal for power and economy.
The secret is to know enough about spark plugs to be able to choose the right type, and to understand what the plug has to say about conditions inside your bike's engine. It is very effective when you get it right.