While I concur that your popet valves may be the problem, I'm assuming you had them on there before and you did not have the idle problem. If I'm wrong, they would be a good place to start, however I have some other suggestions. First, during rebuild, did you follow the instructions on how to properly seat your throttle plates before "nitting" the screws that attach them to the shaft? It may seem like your throttles are closed, but if they are not rotated and centered properly, they will hang leaving a miniscule gap. Backing out the idle screws all the way won't make a difference. Next, make certain you do not have a leak at the throttle shafts. Did you have the bodies re-bushed? Do they need to be rebushed? Most important, review how the throttle shaft seals were put in and make certain all the seals, washers etc are in the right order and seated properly. One last item, one often overlooked: Are your jets centered? Do you have a centering tool to do it correctly? If your needle is hanging on the way down, you should have a rich condition not a speed issue, but a slightly, just slightly hung piston can increase idle slightly.

DS