[Excaliber]

How much advance you run, or NEED to run is dependant on a couple of variables. One would be the cam profile and it doesn't have to be "that radical" to warrant 20 degrees. My cam profile is somewhat modest, 2000 to 6200 rpm. Cylinder filling is the issue. With more overlap at low rpm the spark needs to get a head start to get full ignition. A single plane non heated intake is another potential problem, compounded by running twin carbs. That all combines to make for a "wet" fuel mixture at lower rpm, the fuel droplets aren't atomized as well and remain liquid to some degree. So you need more time to fire the "rich" mixture.

Smaller intake ports or runners, maybe a single carb, give you better throttle response at lower rpm, you won't need as much advance, even with a healthy cam. There is no "one" setting that works for all engines.

I think Don's advice is pretty close when you consider most of us are running a healthy cam and big intake. Tuning of the carb itself will also have an impact. Accellerator pump squirt, spring rate for the vacuum secondary, power valve setting. But timing is FIRST, carb is second.