| I think dual spark is one of those things unique to 2vlv motors - a lots of air-cooled Jap racers in the 70's had ds head retro-fitted - the theory being that it gave a better burn to compensate for the lumpy head/piston design created by two large valves. Moriwaki and Yoshimura did a lot of that stuff - I think there was never anything conclusive to prove the theory though, and it all died out when 4vlv motors started coming online.
For all their good points, the 4vlv Duke motors have their fair share of probs. The 2vlv is a better road engine by far imo. | 
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