double carb setup installed (looking for advice)

What do you have for throttle return spring(s)? It is imperative you have something besides what's on the carburetors if they have any at all. At idle, there is no engine vacuum to speak of against the throttle plates, but at speed, if there's not "enough" throttle spring, the engine vacuum can keep the throttle plates open. Maybe you can post a good up close picture of your throttle return spring setup.