Edelbrock AVS - distributor vacum port

If it clatters with more than 1/2 throttle, you are running too much mechanical advance, way too hot plugs, crappy gas, burning oil, or too rich a mixture. (retard you initial timing about 3 degrees or buy premium).

Too much vacuum advance will cause a light throttle clatter (ping) which you will be able to totally "step through" by 1/2 throttle.

In either case, the vacuum advance is totally out of play by 1/2 throttle.