"My experience with the metering rods is on the 4 bbls, so maybe the 1 bbl's are set up differently. "
Well I found my old '73 Ford Shop manual and the YF carb is a bit different and appears to have a mechanical linkage off of the accelerator pump that lifts the metering rods. (Had I read Joe's post again, I would have seen he had already stated the same thing.) The RBS has the vacuum actuated metering rods. From the reading I did tonight some carbs had 3 steps on the rods to help it transition a little better.
As far as the change in vacuum is concerned I was thinking that if it had vacuum actuated metering rods maybe the forced induction would act on the top side of the vacuum piston and the cross sectional area of the rod itself to keep the metering rod down in the orifice with less engine vacuum.
The accelerator pump usually has two check valves. Just like any positive displacement pump, it will have one on the suction side and one on the discharge side. The ball on the discharge side prevents air from being pulled back through the nozzle when the pump returns to normal position and is drawing in fuel.
The air bleed does several things. It is a source of air in the fuel circuit that emulsifies the mixture before it exits the passage. It also bleeds off some of the vacuum signal to the fuel bowl. A smaller air bleed will increase the fuel metering signal and a larger orifice will decrease the signal. The air bleed is calibrated to provide the proper air fuel ratio at idle. And lastly, it may act as a anti-perculation vent. Without the air bleed, when the engine gets hot upon shut down, fuel would be forced into the carbs venturi as the fuel in the passages is warmed up.
Doug
