A rare but consistent problem I found in 2 600 cfm Edelbrocks was the secondary booster nozzles were not sealiing tight to the carb body. Take the top off the carb and check the 2 screws that hold each of the secondary boosters in place. It seems one or more of the carb body holes aren't drilled and tapped deep enough so one or more of the screws bottoms out before making the booster tight against the gasket and carb body. The screws feel tight but no seal is made. Loosen one screw at a time and try wiggling the booster gently. If it moves, the screw remaining in place is bottomed. If it doesn't move replace the screw and remove the other one and try again.
The fix is very simple. Just put a small flat washer or two under the head of the screw. No need to drill and retap the hole deeper. As further evidence the booster isn't tight you can remove it and study the gasket carefully. A properly seated booster will leave an impression evenly across the gasket. You can reuse both the booster gaskets and the main body to air horn gasket if they didn't tear. I always put a tiny dab of anti seize on all the screws of these carbs.
On one carb all it did was fill the cavities the counter weights swing in. On the other it was loose enough to cause a very serious spillage and over rich running condition as fuel trickled down the secondaries past the closed throttle plates. The 500 and 600 use the same housing and the difference in airflow is due to the 500 having much beefier boosters which restrict airflow.
( see an easy upgrade trick here?? ) Although this is a rare problem it might be a production flaw since the 500 and 600 are the same until assembly.
Just for your info the 2 carbs with the leakage were both purchased brand new around June of last year and from 2 different sources.
Hope this works for you.
Mike
