Were both carbs double pumpers (assumption from the 650 cfm) ? I know the idle circuitry between a 600 and 750 vac secondary carbs behave different. The smaller carb always transitioned and idled better. You need to start with the settings for that list number as suggested and tune from there. Float levels etc. will all come into play. Can you state your engine combo and more detailed info on the carb before and after ? Without knowing your skill level and experience it is hard to help especially without data to draw from. A seemingly direct swap could have resulted in a different vacuum source being used for the distributor and then being blamed on the carb.. I have been fixated on curing a problem, that never was the culprit before. Outside objectivity helps sometimes.
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