Check your carb for proper operation, if it is a Holley 4bbl they had vacuum secondaries and if things aren't right they won't open. Also look down the the primaries with the engine off and open the throttle wide open. The governor set up on these carbs spring loads the throttle to the open position, if the tension spring behind the cover with four screw on the choke side of the carb isn't set tight enough they won't open much. It has a little stud that can be moved to 5 positions to select different spring rates and set the top rpm's. If the primaries don't open all the way the secondaries will never open.

Steve