The engine was designed to run ported vacuum to the distributor, which will only be there at part-throttle cruise. Then it will be somewhere in the 15-18 inch range. As soon as any load goes on the engine the vacuum goes away and the distributor returns to initial + centrifugal advance. No ported vacuum at idle, and slightly less than full manifold vacuum at cruise. Setting the timing to the BB with full vacuum at idle will result in very severely retarded ignition timing and very low power.
Jerry

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