Surging idle is almost always an ignition thing. Double check your timing, and make sure the throttle blades are closed enough to cover the ported vacuum hole. Pull the cap and make sure you haven't lost a spring on the weights and check for free movement of the weights.
curtis73 is on the money but let me add a couple minor points. Surging can also be due to a loss or fluctuation in vacuum from anywhere. ERG valves used to me a culprit when they went bad. So check over all your vacuum related components and connections to make sure they are in working order.
And like curtis73 pointed out, if your throttle plate is not closing far enough to cover the vacuum slit in the throat of your carb, it can cause surging like crazy. One quickie test is to shine a light down the carb with the engine at idle. If you see little droplets of fuel gathering around and then being sucked into the venturi, you'll want to inspect your throttle plate setting. Throttle plate position can be effected by a number of different factors...you'll have to sort through them to find the offender.