If the carb is a q-jet check the power piston spring. If the spring is too strong it will raise the primary jets at idle. More on this in cliff's book.
Make sure the secondary throttle plates are fully closed at idle. The secondary plates are closed by a coil spring on the secondary shaft. Just rebend the spring so more force is applied to the shaft. This is the problem I had on my q-jet & it took a long time to find it.