The vacuum advance mechanism should be getting be getting ported vacuum from above the throttle plates, if it is hooked up to manifold vacuum it will cause part throttle detonation. Check your vacuum source at idle, if it is getting vacuum you have found the problem. There should be a hook up on the carb for ported vacuum.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo