The idle issue is the total lack of intake velocity. Picture it this way: Hook up your shop vac to a piece of 2" pipe. The air will be sucked through the pipe at a very high speed. Now replace the pipe with a 10" pipe. Same amount of air goes through, but much slower. Drop a hand full of rice past the front of the 2" pipe and it will get sucked in like a shot. Drop the same rice in front of the 10" pipe and about 1/2 would fall right by, and the other 1/2 would drag across the bottom of the pipe until it got to the hose where the air velocity picked back up. When the air goes too slow through the intake, the fuel falls out and puddles on the floor causing terrible idle problems, lots of unburned fuel fumes in the exhaust and a tendency to load up at anything less than 1000 rpm. The intake you have there is cavernous.
Now also consider the fact that the air is being pulled through 8 holes instead of 4. Even at idle, those carbs are feeding from all 4 holes each. The amount of air being pulled though each one is 1/2 that of a single carb setup, resulting in much less responsiveness to whatever tuning you try to do. The fact it is a 512 really helps since it pulls more air per revolution, but it is still a LOT of carb for the motor. My suggestion was for tuning purposes on the motor as a whole, switching to a small single plane manifold or a big dual plane with just one carb would make your life way simpler until you get the car sorted out enough to need (or at least handle) all that intake capacity.
