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My guess is that the vacuum pot on the distributor is leaking. The vacuum measured at the carb source is much too low to "finger test" as the maximum available there is around 2"hg, not the regular manifold vacuum of around 18-20"hg. The vacuum provided by these carbs is a mixture of venturi vacuum and manifold vacuum and the "mixing " is controlled by passages within the carb and a valve on the front right side near the vacuum port. All pre-'57 Y blocks used these loadamatic distributors with ONLY manifold-venturi vacuum to provide the spark advance and as well, the carb and distributor must match to function correctly. There is NO provision for centrifugal advance. If the vacuum pot is leaking or ruptured, the engine will get absolutely NO spark advance , run like a dog as well as overheat due to retarded timing. I fixed my 54 Merc 256 with a Holley 390 CFM carb and a modified '57 distributor. Good luck : Bruce