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Al,
With a DGV, does the ported vacuum depend on flow just through the primary barrel, or both? Could it be that you're still having some vacuum advance with the secondary open? I really don't know how that works with that model carb...
If it's NOT that (vacuum is controlled solely by the primary), I'd suspect your DGVs are going lean. I know nothing about your motor in particular, but your first step is to get both carbs into absolutely perfect synch, checked with an airflow meter -- there are adapters that fit DGVs; do a search for 'Synchrometer' and you'll come upon it. Nothing good will result without that. (Pardon me if I'm preaching to the choir here; I don't know what you've already done or how.)
Select the primary idle jet that gives you best idle with the mixture screws 1.5 to 2.5 turns out.
Select the smallest primary main jets that don't bog coming off idle, or surge at mid-RPM.
Typically, the secondary main jets will be one size SMALLER than the primary mains, although there are enough variables that this is only a recommendation for a starting point.
Finally, select the samllest secondary idle jets that don't bog at the point the secondaries just start to open.
So, if the ping is with the secondaries almost closed, up the size of the secondary idles. If it's when they're halfway open, up the secondary mains and see what happens. If that works, you may be able to go back down a step on the secondary idles.
Not sure what the A/F meter you mentioned is about (O2 sensor?), but I'd go with two EGT sensors/gauges, which are not as expensive and provide info that's equally or more useful. (Again do a search.) Any motor will have a particular exhaust gas temp at its most efficient/powerful mixture and timing. If you can establish what that is for your motor (a dyno helps here, but...), you can tune for that temp over a wide range of RPM and loads.
Cooler than optimum = too rich / hotter than optimum = too lean. Then it's a matter of using all the timing advance you can, just short of pinging. More is better, under all conditions and speeds. Always.
Max advance will allow you to use a slightly leaner mixture to achieve the optimum EG temp. At that point, it's a matter of going back and forth making small adjustments to both mix and timing until you nail it or fall over from exhaustion. And then the weather changes and it'll be a bit off anyway; don't go nuts chasing that last 0.82 HP.
No secret to any of that... all the rest is the physical construction of your motor, and I have no idea about that for Datsun.
Or you can just forget all that and try running plugs one or two steps cooler than the factory recommends. That'll reduce the pinging, at least (not kidding).
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