There is a difference in setting for the highest vacuum versus setting for the highest RPM.
Historically, there were some low compression engines (not that the stovebolt is a high compression engine, but it better be better than 4 1/2 to 1
) in the 'teens and 'twenties that DID have the idle set to the highest vacuum. Once a "rule of thumb" is established, one almost has to break the thumb to change the rule.
If one examines the throttle body on most modern (1935 or so and newer) carburetors, one will find two idle ports: (1) the round hole through which the idle mixture screw can often protrude called the lower idle port, and (2) the idle transfer or transition slot which is just above and just covered by the throttle plate.
What condition of the throttle body will always give the highest idle vacuum? The highest vacuum will be acquired when the throttle plate is completely closed. Opening the plate lowers the vacuum.
With the throttle plate completely closed, ALL of the idle mixture will flow through the lower idle port. Why is this important?
One of the tenets of Carburetion 101 is that there are three ways to increase fuel atomization: (1) add heat, (2) increase the air velocity, and (3) add additional fuel. Since the throttle plate is completely closed, there is virtually zero air velocity in the intake manifold; so ADDITIONAL FUEL MUST BE ADDED. Again, with the absence of air velocity, some of this additional fuel will "puddle" in the intake manifold. Now when the throttle is opened (to pull away from a stop sign), the increased air velocity in the manifold sweeps all of the puddles into the cylinders, and there is an instantaneous overrich condition which causes a hesitation.
But the fuel necessary to keep the engine running BEFORE fuel flows from the accelerator pump comes from the idle transfer slot. But there was no fuel flowing through the transfer slot because the throttle plate was closed. So right after the instantaneous overrich condition, there is now an instantaneous overlean condition before fuel flows from the accelerator pump, and the hesitation continues. We are talking milliseconds, but the hesitation is noticeable. And the owner will curse the "defective accelerator pump"
Note that this hesitation will occur ONLY when the engine comes off idle, and will occur even with a new accelerator pump. In fact, literally millions of good accelerator pumps have been replaced (at the owners expense) because of this simple adjustment.
Just to take this thought a bit further, look at the two and four barrels GM used beginning in the mid-1950's (compression was increasing). The idle was set with an "air screw", that basically was an adjustable vacuum leak!
A few thousandths opening of the throttle plate will cause the vacuum to be a bit lower than maximum, and will minimize or eliminate the off-idle hesitation.