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Al,
With Weber DCOEs, there's no option other than hooking up to the manifold. I was reluctant to do that at first, but did some research. Consensus seems to be that hooking up to ported vacuum is somewhat old fashioned, and hooking to the full manifold vacuum is actually more "correct." This does make sense, as the manifold vacuum is going to track the mass of the intake charge - and its effect on cylinder working pressure - better than airflow outboard of the throttle plates.
I had to get over the idea that full vacuum advance at idle was a Bad Thing, and that the advance should fall off when accelerating from idle. In all other motors I've owned (not just Volvo), I've either used pure centrifugal dizzies, vacuum/centrifugal hooked to the ported vacuum, or even pure vacuum hooked to the carb port. All of those advance coming off idle, but that's actually not efficient at idle. I got past my initial mistake of retarding the basic timing at idle pretty quickly - let it be 30 BTDC with vacuum.
How the advance tracks has a major impact on carb jetting/tuning, so experimenting with hooking it up to a port vs. the manifold doesn't tell you much unless you completely retune the carbs as well.
Combustion chamber shape and squish characteristics have more to do with the motor's resistance to pinging than the vacuum setup, but it's the vacuum that helps it get surprising fuel economy.
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