Vacuum Secondaries

The "right" size carb will vary depending on the use of the engine. That's why 600, 750, 850, and 950+ may be the "right" carb. One needs to know how to tune them. Then match the combo and use with the size. The typical math basing cfm on displacement and rpm required really has very little to no bearing on sizing a carb. Especially when some carbs are dry flow rated, some are wet, and 2bbls are tested differently.