Jean Fouche wrote:

        Let me give it my best shot.

         The location of the 12A sparkplugs are different on the rotor
housings. The exhaust ports open sooner on the 12A twin dizzy. The inlet
ports were big enough for adequit breathing. The apex seals are 6mm carbon,
not good for longterm durability but excellent for high performance (past
9000 rpm). The 12A twin dizzy engine internals has less rotating mass and
could rev cleaner and higher than the standard 13B. Overall the twin dizzy
12A was designed to run without the pesky emission controls in mind, or at
least up to 1972. The leading and trailing distributor had different (and
individual) mechanical and vacuum advance curves. The exhaust manifold was
cast iron shaped into tubular exhaust extractor manifold.

         The 13B, or at least the one we saw in South Africa was sold with
thermal reactor etc. The inherent design of the 13B was to run on emission
and therefore all working components was a compromise in this regard. The
apex seals were 3mm two piece steel, much better for durablity and high
mileage.

        Hope it helps
        Jean Fouche

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