When setting up a rochester, why is it so important to know manifold vacum at idle??
A higher manifold vac. at idle has more of a pressure drop [as it is known] This pulls on the idle circuit and is what initiates flow of the circuit. Less vacuum means less pull but the engine still may need the same amount of fuel [or more]. This requires idle passages to deliver more fuel therefore (to keep the air fuel ratio close to where complete combustion occurs.) BTW, Q-jets have very sensative Idle and low speed circuits due to thier design. [double venturi] Often times not even requiring any "accel pump fuel" to start engine. This is the reason that the calibration is so important on the idle. The factory calibrations (some) had a very "narrow" window when it came to adjusting idle. [lots of emissions standards implemented] When you custom tune a Q-jet idle circuit, you are actually widening the adjustment range also.
{I think all this is correct and is to the best of my knowledge}
