Any idea why would anyone add a second spark plug in a B25 head?
https://www.ebay.co.uk/itm/275607378225?hash=item402b7ad931:g:LwEAAOSwPaljuBSN&amdata=enc%3AAQAHAAAAoLvQcO97EbS6nuleRvZ3f1%2B2RYwggYclBaPqLGxo9w9086%2Fe4O%2BtV3oMHTy5IuXhqFT4X1DnT7O%2Fwc2sBPS7A3IdbU51S59Sw0KPl7O1vQ1RiWRsivkSmL9kR6g97OMZzFbARkTKrfI0b0KURXrkYFfFN9onElRKmhxMX9liiaDsgpYXjfBJX5XqUQZ2HZdPMTKN1moXzZ2cLBHoCpX6JXg%3D%7Ctkp%3ABk9SR-bw3dW0YQ
it was standard practice on 2 stroke race bikes, you would use 1 plug to get to the line and then as you had to stop the engine, swap over to theoretical clean plug, and use that for the race. probably ideal for a harder race plug and softer warm up one.
an alternative thought would be to re position the spark for the best through flow, another race mod.
Hi,
I think that in a 4 stroke it is to ensure complete combustion in the cylinder. With high compression some of the mixture can get 'trapped' on the side away from the plug as the 'flame front' has to travel over and around the dome of a high compression piston.
This was quite common on Triumph twins. One central one would probably do but there is very little room between the valves.
Dave.
My BMW F650GS 650 cc single had two spark plugs and was referred to as the twin spark version. It provided a better and more complete combustion performance than the older single spark version of the Rotax engine and probably, more importantly, helped BMW to meet the EURO 4 emission levels.