Fresh carb kit? Top cover gasket in good condition? You might try using a fibre washer on the center bolt on the top cover to insure there’s not a vacuum leak right there. A vacuum leak at that point will keep the power valve from working as intended. Load-O-Matic distributor? If so, is it advancing correctly when checking with a timing light? Also check that the pair of air bleeds located close to center carb stud on the top cover are clean and/or clear. Shooting some carb cleaner into these with the engine running should indicate if these passages are clear or not.