If the engine has not run well since the rebuild, it is possible the carb is not the problem.
Start with a compression test (even if the cylinder head is new!).
Next would be hook the engine up to a diagnostic oscilloscope to look at the ignition.
If neither of these tests turn up anything unusual, would then look at the valve timimg, and valve lash adjustment.
An engine needs compression, fuel, and spark; all delivered at the correct time, and the engine runs. The trick is to determine which item is giving the problem. Of these, the carburetor (even a Rochester B) is the least likely to cause problems.
Jon.
