Multi-colored deposits on the insulator and electrodes indicate the presence of a large number of additives in the fuel or engine oil. Black, matte carbon deposits on the insulator, electrodes and housing are a sign of operation in most modes on an over-enriched mixture or too high a glow number. It is necessary to check the carburetor or injection system settings (for example, by gas analyzer) or install a hotter spark plug. Shiny black carbon deposits indicate that lubricant is getting into the cylinder through the piston rings and valve guides. Engine overhaul is required.
If the insulator is "snow-white" - it means that the spark plug was operating at the maximum permissible thermal mode. Reasons: too early ignition, too lean mixture or too "hot" spark plug for this engine. It is necessary to check the carburetor settings, the characteristics of the automatic ignition advance and, if they are normal, install a "colder" spark plug. Well, the next degree of spark plug overheating is melted spark plug electrodes (it's good that the piston bottom isn't burnt out).
Sometimes it may turn out that all the spark plugs of the same brand in different cylinders have different colored insulators. There may be several reasons. As a rule, in carburetor engines the mixture (by composition) and anti-knock additives are distributed unevenly throughout the cylinders.
There are engines with different values of compression ratio for each of the cylinders. And between engines of the same model, the compression ratio sometimes differs by a whole unit. Due to deviations from technological processes, materials, etc., there is also a large spread in actual heat ratings among spark plugs of the same brand. With a large difference in the condition of the spark plugs, it is sometimes necessary to install spark plugs with different heat ratings even in one engine. In order to always have a reserve in heat rating under different operating conditions, it is advisable to have two sets of spark plugs and install "colder" ones in the summer and "hotter" ones in the winter.
Recently, a number of companies have been producing spark plugs with an extended thermal field of the spark plug. For this, the central electrode is made of copper, and a coating of heat-resistant materials is applied on top (nickel, chrome). Sometimes the side electrodes are made of metal with good thermal conductivity and a heat-resistant coating. Another original solution is to make a miniature central electrode from platinum that does not protrude from the insulator. Especially "cold" spark plugs with a heat rating of 300 and higher for forced engines (turbocharged, two-stroke, etc.) are usually made with a small diameter silver electrode and a short insulator skirt. Before installing such spark plugs, it is sometimes recommended to first warm up the engine with hot plugs and only then install cold ones.
It is necessary to keep in mind that when cleaning the spark plug in a sandblasting machine, the surface of the insulator is destroyed. It is better to turn the spark plug over and pour solvent, gasoline into the housing or use a special aerosol can. Then clean the insulator with a wooden stick and blow it with compressed air.
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