that's normal. one port, (the one that has vacuum at idle) is tied directly to manifold vacuum. the other port is carburetor vacuum, so there is low vacuum there until you rev up the engine. Of course, when you rev the engine, the vacuum on the other port will go lower, as manifold vacuum decreases when accelerating, esp under a load. I prefer the carb vacuum for vacuum advance, although there are two schools of thought on this. Try 'em both, see which one you like the best.
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