The problem is the B model carbs- - - - -they're WAAAAAY too big for your application. When the carbs try to go from the idle circuit to midrange, the velocity in the venturis is so slow that you're getting nothing but air, no fuel. Unless you really sneak up on the throttle opening, your flat spot is the result of momentarily starving the engine for gas. The only solution without changing carburetors, and this is a "aspirin for a brain tumor" type of fix, is to give it a massive squirt of raw gas from the accelerator pumps and hope enough of it evaporates to hide the lean spot. The correct fix is to use carbs with very small venturis, so splitting the airflow between two carbs doesn't kill the velocity of the air through the venturis. You will still increase the maximum airflow at full throttle, (the only time where the dual carbs are any advantage in the first place), without destroying the off-idle and midrange performance in the process. Find a matched set of Carter carbs designed for the 216 engine, and build them up exactly equal in air and fuel flow. The RIGHT fix is to throw the dual-carb manifold as far as you can, and go back to what the GM engineers designed for the engine- - - - -one carburetor!
Jerry
