Swedefish, it sounds like you are chasing a few problems. Let's take this slowly, and with as much order as possible: What was the needle doing on your vacuum gauge? Was it floating slowly between the upper and lower readings? Was it vibrating rapidly between the two readings? The action of the needle is as important as the numbers it points to. What is your altitude? This will also affect the engine's ability to run. The 600 CFM carburetor should work fine, but run out of breath above about 5,500 RPM. A common problem with carb installations is the base gasket. Is your gasket installed properly? Are there any mis-matches between the gasket an the bottom of the carb? How about the top of the carb flange on the intake manifold? I have seen a spreadbore gasket installed under a squarebore carb before- boy did that run like a dog. I have also seen the gasket installed backwards, and sideways (once). Are all vacuum ports on the carb sealed or attached to components that they should be? Your metering rod pistons should be all the way down at idle, and the springs that came with your carb are not going to allow this at idle with such low vacuum. You really need to drill a hole in each of the primary throttle blades, and do it like Chryco suggested. While you have the carb upside-down for drilling the throttle blades, adjust your idle speed screw until the blades are just above the idle fuel port in the venturii. After you have done these things, put the carb back on, and adjust the mixture screws out 2 turns from seated. This will give you a good starting point for your further adjustments. A good way to check the need for additional airflow at idle (without removing the carb or drilling the throttle blades), is to open the secondary throttle blades, to allow more air into the engine without moving the primaries with the idle speed screw. If the idle speed picks up, and smooths out, you need to drill the plates. Keep your initial timing at between 16-20 degrees, and adjust your carb for best idle and highest vacuum. After doing this, it is safe to play with your timing, to get the proper advance curve and total advance number. I think that there must be something simple that you are overlooking, and it is frustrating you without letting you know it's wrong. Raising the float level from a measured 7/16" to about 3/8", is a good idea as well. I hope this helps a little.
