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I'm sure you've all been there but I had a sleepless night last night, mainly with my brain popping up random thoughts that i couldn't get rid of....and it kept me awake! One of them though was "Why DID early Yams have twin spark plug holes!!?" I came to the conclusion that it was either for 'Performance' or 'Diagnostic'...... "Performance" to make it better or worse! or "Diagnostic" to see what was making it better or worse! yup not the best discussion to have with yourself at 4 am....but.... It also got me thinking about what was available back in the 70's to do either of them...As regards to 'Diagnostic', there was Colortune sparkplugs, which were a popular 'accessory' that found their way into many a christmas stocking for dad who had everything. It was a plug with a glass inspection construction so you could 'see' the colour of your spark!.....oh joy!....come on... it was 4am!! but would Yamaha construct cylinder heads just for this purpose?....unlikely I thought!...unless it was a high performance engine......not your run of the mill Trial bike... it was probably 04.15 by now! Which made me think of 'Performance'. Was there really a need to equip their cylinder heads with dual spark plug holes to run twin plugs? Presumably that would mean twin HT Coils and maybe twin contact breakers? makes you think doesn't it? it was probably at least 04.30 by now! I don't recall my mate who had twin plugs on his AT ever having twin coils, true he had both spark plugs inserted but only one was ever connected at a time. He would change the HT lead over to the other plug if it got too fouled up...but thinking about that today (04.40) the chances are the carbon fouling was probably all over THAT spark plug too!!....but heh we were just 16!...it seemed to be the 'right thing' to do, swapping HT leads.....wow...the performance change...Not!! Anyway, (04.45) I came to the conclusion that dual plug holes was probably related to performance more than anything else......mass produced?...domestic market vs sports market?.......then just blank off the hole that's not in use and make use of the twin holes for the sports related performance. Seemed logical, and probably not untypical of 70's thinking. No I didn't drop off ...I still had Quantum and String theory to discuss with myself along with what am i going to do today?, what I'm going to be ABLE to do today with so little sleep!!.... what's that ringing?...it's the alarm......time to get up! Then I thought "Why don't I google it? " Twin spark plugs on Cylinder heads.... I was right after all....its performance related... https://www.rediff.com/getahead/report/slide-show-1-biking-and-motoring-why-twin-spark-is-superior-to-single-spark-tech/20140104.htm So I just have to answer one final question.... Why didn't i get up at 4am and look it up? ...would have saved myself all that 'deep thinking"! Gee ...i'm tired!!!