[41ford1]

What are size are the main jets and power valves? What manifold are you running? Are you running straight or progressive linkage? Progressive linkage can be hard to set up correctly. #47 jets and 3.5 power valves should be a good start for straight linkage. I know more questions to be able to better answer yours.
You might have a vacuum leak that is some what corrected when your hand is blocking the front carb. If you are running a regular dual manifold (Offy) try blocking the rear position. Then tune each carb on the front position. That should give you a decent starting point and show any carb problems. Good luck and keep us posted.