Honda CB125 CB160 (1972) [95/139] Fig 5 30

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89 5 3 IGNITION CIRCUIT by combustion If lead oxide adheres to the igni tion section it becomes a conductor and consumes a part of the voltage The phenomenon described in the previous section is generated The spark plug should be constracted in such a manner that it will reduce such phenomenon as much as possible and the insulation and electrodes should not be subjected to the effects of chemicals 2 Spark plug construction The general construction of spark plugs employed in automotive vehicles is shown in Fig 5 29 The center electrode Fig 5 30 is wrapped by an insu lator Fig 5 31 and the exterior surface is wrapped by metallic tube Refer to Fig 5 28 thru 5 31 3 Heat characteristics of spark plugs The heat characteristics of a spark plug and quality will determine the performance of the plugs as well as the performance of the engine a Spark plug performance Fig 5 32 Favorable status of plug The ignition section of the plug fitted to the cylinder head is hindered by residue generated from combustion gas or oil entering the combustion chamber during operation of the engine This ad hesion acts as conduction on the insulation and provides a shorting passage for high voltage current resulting in a weakened spark and lowered output of the engine or complete stoppage of the engine To prevent such occurrences the surface of the ignition section should be heated to an extent that the temperature burns the residue approximately 450 to 600 Q However temperature differs in accordance to Fig 5 33 Overheated plug the engine condition Such temperature is termed Selfcleaning temperature However if the ignition section of the plug is overheated the ignition section becomes an ignition point resulting in pre ignition and inducing knocks which lower engine output Hence it is necessary to keep the temperature of the ignition section below the pre ignition temperature which is appro ximately 800 C However this varies according to engine conditions The ignition section of a plug should not be over heated or overcooled Fig 5 34 Smoke plug Wet left Dry right

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