I should add a little more clarity to my last post. Manifold pressure will drop as you open the throttle. balancing the carbs pneumatically only means that the butterfly valves are operating exactly together. With the butterflies nearly closed, balancing makes a big difference in air flow if one butterfly is set even slightly different from the other. The wider open the throttle is, the less effect balancing has on performance. Your problem sounds very much like an obstruction in the fuel line, jets, or galleries in one carb. The fact that it happened suddenly makes me suspect trash.
Bill.
