They call this "hunting". And is usually caused from not having all of the data tag settings exactly as specified on both the carb and ign timing. Normally, if you follow all of the steps on the emissions label you won't have any trouble.
It is caused by the the computer trying to retard the timing for smoother idle. The computer gets it's information from the "3 and 10 inch vacuum switches" mounted on the valve cover or near the firewall. If you un-plug the switches, it should stop. Or you can just remove the vacuum lines on the switches and cap them off . The amount of timing retard that the computer controls is insignificant, and only brought in during idle situtations.
Now is a good time to do the H.E.I. swap, since you have changed your carb.