In the early days of motoring some spark plugs could be dismantled for cleaning, 'progress' means all modern plugs are all one piece.
Dirty or fouled spark plugs can cause a misfire or a cylinder filing to fire at all. . These oiled up plugs were removed from a running Rover V8 engine (Rover P5b) that constantly misfired. The colour of a plug can help diagnose the mixture in an engine and indicate faults,a light brown normally indicates an engine in good health running a correct mixture.Black plugs indicate a rich mixture (or maybe a stuck choke) white plugs can indicate a weak mixture and/or overheating. Oily plugs can be caused by bore or valve seal wear,if slight it may be cured by running with a 'hotter' plug.A 'hot' plug usually has a longer nose,it is designed to project further into the cylinder and run at a higher temperature,sometimes this results in the plug tip being able to burn off slight deposits of oil.
The ends are covered in black soot.This can be caused by a rich mixture,using the choke for too long, a badly choked or dirty oil filter or maybe an engine that fails to get to full operating temperature . The loose soot is easily removed with cellulose thinners or petrol ,soak the plugs and dry them To remove the baked on residue under the soot,a strong oven cleaner (I used CIF professional when the wifes back was turned) dissolves these baked on deposits.Oven cleaner doesn't do your hands any good so use gloves. A thin piece of wood will scrape round inside the plug around the electrode, metal scraper or a wire bush can leave fine metal particles behind which will encourage shorting out.
Shaking the cleaner out onto a plastic surface removed this oily baked on mess from deep inside the plug,the colour of the clean central insulator can be seen.The plug was then washed in water and dried.
The plugs all cleaned up well,after they were refitted to the engine it started instantly and ran without any misfire. The soot coating the electrode can conduct electricity,rather than jump the spark plug gap the current may goe to earth via this coating and so there will be no spark. If the spark plugs continually fould and they are correct for the engine the cause needs to be traced.
Spark plug sizes.Spark plugs are measured and identified by the dia of the threaded part that screws into the cylinder head,the common sizes are 10,14 and 18mm. The most common for cars is 14mm. The smaller ones are found on motorcycles,chainsaws,small generators,strimmers etc .
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Baconsdozen Imperial Tools
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