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Thread: plug readings

  1. #1
    xplayer2885's Avatar
    Join Date
    Sep 2006
    LI, NY

    plug readings

    WHo has ppics of a lean plug, rich plug an a just rite plug. AN tell me what part of the plug too look at to indicate this. I wanna make sure im doing it correctly. Thanks as always!

  2. #2
    Connecticut CrazyA's Avatar
    Join Date
    Nov 2005
    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.

    A tan colored plug means that the engine is running normal and the air/fuel mixture is correct. This is the correct color a spark plug should be and it tells you everything is fine with the engine. You would install a new properly gapped plug. When installing a new plug, replace the old one with the same heat range. This plug shows normal wear in the center electrode. A new plug would have square edges that helps the plug fire better.
    This plug is worn out from being used for a long period of time. Notice how the center electrode is round and worn from use. A spark plug that is worn takes a lot more voltage to fire and can cause poor engine running.
    This plug shows what can happen when something hits the spark plug. Something from inside the engine has hit the plug and this problem must be repaired before running the engine further. Make sure the spark plug is the correct length for the engine.
    Excessive detonation has caused the porcelin on this plug to break away. If this engine is allowed to run, engine damage can occur. Make sure the fuel octane is high enough for the engines requirements.
    A white colored plug is caused by engine overheating. Failure to repair this engine will result in severe engine damage. Common causes for this are:
    Incorrect spark plug (too hot heat range).
    Low octane fuel.
    Timing is not set properly.
    Cooling problems, (dirty cylinder fins, no or low water if water cooled, low or no engine oil).
    Carburetor air/fuel mixture is too lean (too much air).
    Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines.

    This plug has ash deposits which are light brownish deposits that are encrusted to the ground and/or center electrode. This situation is caused by the type of oil used and adding a fuel additive. This condition will cause a misfire. This can be also caused by changing oils in midstream.
    This plug is oiled fouled, caused by poor oil control.
    Pre-ignition, which will usually look as a melted center electrode and/or ground electrode. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
    Sustained Pre-ignition, which will usually look as a melted or missing center electrode and/or ground electrode as well as a destroyed insulator. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
    Splashed deposits look as if they are small islands of contaminants on the insulator. This is usually caused by dirty carburetor bores or air intake.
    A black dry fluffy colored plug is caused by deposits from a carburetor that is running too rich (too much gas), or excessive idling on some engines. Black smoke coming from the exhaust is a sign of a rich air/fuel mixture. The rich air/fuel mixture must be repaired before installing a new spark plug. Common causes for a rich air/fuel mixture are:
    dirty air filter.
    air mixture screw or carburetor needs adjusting.
    choke is sticking.
    carburetor float height is out of adjustment or float is sticking open.

  3. #3
    Water Bum Rodneyae's Avatar
    Join Date
    Dec 2006
    Roanoke, Va
    Great job on explaining it!!!!

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