Put a vacuum gauge on the spark port of the center carb. If you have any vacuum there at idle your throttle blades are not closed enough. When vacuum drops to zero there your idle circuits will work well.

Bigger than stock cams need more air and or timing at idle. The first thing you need to do is find out how much initial timing you need for your engine.
More timing will increase the idle speed and allow you to close the throttle more.

For example. My 440 six pack has 26 degrees initial and 36 total at 3000rpm. It has a 237/242 @ 50 duration solid cam and I set idle speed at 900rpm so it runs nice and the alternator actually does something at that speed too.

If increasing the timing does not give you enough idle speed to get the center carb throttle closed then you need to start playing with the outboard carbs idle circuits or (last resort) drill a couple of 1mm holes in the throttle blades of the center carb.