Geoff;Have you had your existing distributor apart enough to see the mechanical spring loaded advance cams? If not, remove the points and the points' mounting plate and check the movement of the spring loaded cams. The cams should move easily by finger pressure and quickly return to their resting position. The pivot points of the cams are lubricated and it is possible that the lubricant has dried out. This can happen on both conventional older design distributors and the newer electronic distributors. It shows up as knock or ping under acceleration. Check to be sure the vacuum advance and vacuum lines have no vacuum leaks I had a '73 model with electronic ignition which would act that way. I eventually discovered the problem with the cams when I started to replace the pick-up reluctor mechanism and star wheel on the distributor shaft. Before replacing the parts I decided to clean and re-lubricate the cams and it corrected the ping/knock problem. I went ahead and changed the reluctor element and the star wheel on the distributor shaft anyway since I already had the parts on hand.
The car I mentioned above had the exact symptoms you described - pinging under moderate acceleration. If you really got on it, the pinging would go away. It pinged just when you did not want to put your foot into it, just normal driving. It is a subtle condition and difficult to analyze except by just removing that upper plate and trying moving the counterweights/cams to see if they move freely and quickly, not slowly (caused by dried out or gummy lubricant). I hope this helps.
Joe
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