[Brian]

I set my crab distributors up on a timing fixture; this sets initial advance, also the dwell, both individually and combined. But doesn't do anything about advance curve.....
So, in actual road testing, at certain RPM's/ load, the engine detonates to hell.... Not just the current engine, all the engines I've built and run for the past substantial number of years.

Common denominator in all of these has been my ignition; crab distributors, the same units, rebuilt and set up by yours truly.
So....whilst driving home the other week, after attending a swap meet, listening to the cacophony emanating from the engine, it dawned on me that the dissy was advancing too fast, too soon under certain load conditions. So wondered how I could slow down the advance curve, thought maybe I could drill some holes in the advance weights, which would tend to make them fling out a tad slower....
Next day, got onto it...pulled the dissy, stripped it....playing with the cam/advance/retard assembly, it was apparent that there was bugger all spring tension exerted on the weights; consequently, centrifugal force would cause the weights to fling out to full advance pretty damn quickly.
Another crab gave same results.
Next I stripped a divers helmet dissy with '68' guts. This runs the same size 'small cam' as the crab.However, it definitely had a strong spring tension exerting pressure on the weights. Cool!! That'll slow down the advance.

So, I built up a crab dissy using the '68' cam/weight assembly. Initial testing shows a definite improvement; I still get pinging at certain speeds/loads, but it's much better.
Still room for further investigation/playing, thought I'd put it out here, see what the experts can suggest.