Hi there. Just my 2 cents. If you have a carb system, like the Jab, there is no return line. You only need a return line in a fuel injected system because it operates differently and at a higher pressure.
The typical carb type set up only uses about 3 to 5 pounds of fuel pressure to the carb or carbs. The fuel is only used as the engine runs through its rev range. At low revs it doesn't need as much as at higher revs/power settings it uses more fuel.
Try to think of the carb float bowl like the toilet cistern at home (it is the same principle). When the cistern is full, no water flows into the bowl, even though the water pressure in your plumbing is always there. Release the water and the float drops,
the valves opens until the cistern is full again.
In your carb/ fuel pump/ set up, once the float bowl in the carb is full, the fuel flow stops until the float drops and allows more in. This happens constantly as the engine runs. The fuel pump is set at a low pressure and once this pressure is reached, the fuel pump is idling (the fuel bypasses internally in the pump until the pressure drops again. When it does, the fuel starts flowing to fill the float bowl again). So the amount of fuel passing through your flow meter should not change as long as there are no leaks etc. The boost pump in this set up is also low pressure, so that it doesn't overcome the float valve pressure setting or the engine driven fuel pump setting.
In an injected engine, the fuel is delivered at around 38 to 43 pounds,usually to a fuel rail. The injectors are attatched to this rail and the amount of fuel allowed into the engine is determined by engine load through either a vacumn arrangement on the older systems or by electonics as in the new type of system. The fuel pressure is controlled by a pressure regulator at the end of the fuel rail and the excess fuel is sent back to the fuel tank. This is why the injected type engines MUST have a fuel return line .