Holley's Double Pumper carburetors are a popular choice of four-barrel carburetors amongst hot-rodders and classic car enthusiasts. The Double Pumper carburetors have a mechanical link from one set of two venturis (or "barrels") and the second set of two venturis. This provides crisper throttle response during acceleration. The difference between the 650 and 750 is size. You need to understand a little about carburetors to understand this difference and how it's measured: in CFM or "Cubic Feet per Miute" of air volume.
Venturis are cylinders cast into or bored through the carburetor's throttle body. The size of many carburetors -- single motorcycle carbs for example -- are measured by the diameter of their venturi. Automotive and multi-barrel or multi-carb stacks would have to be multiplied if they were expressed in size. So, their measurement is expressed in the maximum air volume that can flow through the carburetor at wide open throttle.
The function of a carburetor is to mix gas and air, delivering precisely the right ratio and quantity at any given throttle position. The throttle position actuates butterfly valves in the Double Pumper. Other carburetors have various other mechanisms to open, from slides to rotary valves. The Double Pumper's butterfly valves are completely closed when the throttle isn't actuated; it relies solely on its idle circuit. As the throttle is actuated, the the butterfly valves open. When partially opened, they limit the volume of gas/air mixture that passes through the venturis.
CFM is measured only at wide-open throttle, when both banks of butterfly valves are open, letting air pass through all four venturis or "barrels" with little obstruction. Generally, the larger the venturi(s), the greater the air-flow at wide-open throttle. If you have two, otherwise identical carburetors, like the Holley 650 and 750 Double Pumpers, venturi size will directly indicate the amount of airflow.
When translated, larger venturis, means more air flow at full throttle. Carburetors are rated by the maximum amount of air that will pass through them given a constant amount of vacuum. The 650 Double Pumper flows 650 CFM and the larger venturis of the 750 Double Pumper flow 750 CFM. Bigger, however, is not always better. Maximizing power and throttle response has many factors. Generally, it is the correct mixture of gas/air for a given condition. If the engine can optimize more mixture, the 750 will increase the engine's maximum power at full throttle.
