TC - a well broken-in 216 or 235 will idle with the throttle plate closed with either a properly restored Rochester or the W-1.
As I have never personally adjusted a 261, I cannot say.
It is rare today to find a workable Rochester, although they do exist. Issues include the well-known warpage, but just as bad is the mixing of parts of different carburetors by the commercial rebuilders. If I were going to use one, I would not set for the highest vacuum.
Most believe that a hesitation off idle is always a faulty accelerator pump; when in fact, the off-idle or idle transition circuit (call it what you wish) on a carburetor with a properly adjusted idle actually provides fuel to the engine for a fraction of a second PRIOR to the fuel provided by the accelerator pump. As the lower edge of the off-idle slot would be covered by a closed throttle valve, no fuel would flow through this circuit, and the entire idle fuel/air mix would be forced through the lower idle port. As mentioned above, this causes "puddling".
As engine displayment grew, and camshaft duration became larger, less of a chance that the engine would idle with the plate completely closed. In fact, a longer camshaft duration would cause the throttle to be opened so wide that the vacuum signal at the idle ports would diminish (low air velocity past the port) to the point where some carburetors had factory drilled holes in the throttle plate(s). As this would add air to the idle, the throttle plate(s) could be adjusted closer to the throttle bore, thus increased air velocity resulting in a better vacuum signal in the idle ports.
There is really no rule that works in all cases.
GENERALLY, to set the idle on a relatively stock engine, I use the following procedure:
(1) look up the factory recommended "range" for idle mixture screw adjustment
(2) divide that range in thirds.
Where I set the idle mixture screw then depends on the condition of the engine.
(1) freshly rebuilt up to maybe 500 miles - 2/3 of the range
(2) engine with 500+ miles up to ??? but still with good compression and not burning oil - 1/3 of the range
(3) worn-out engine (low compression, and burning oil) - the upper range limit
Examples:
Consulting the manufacturers specifications for an original carb shows an adjustment range of 3/4 turn to 1 1/2 turn. Divide the difference by three gives 1/4 turn.
So 1/3 of the range distance would be 1 turn (3/4 initial + 1/4)
2/3 of the range distance would be 1 1/4 turn (3/4 initial + (2) x 1/4)
The full range adjustment would be 1 1/2 turn (factory spec).
So in my examples
Freshly rebuilt - 1 1/4 turns
Good engine broken in - 1 turn
Worn-out engine - 1 1/2 turns
Once this setting is complete (it takes a lot more time to explain than to do), I would run the engine at a high idle until normal temperature is obtained, and then set the curb idle RPM with the throttle positioner screw.
Finally, I would now use the vacuum gauge to verify that the idle vacuum is within the expected range for the engine tune (read camshaft duration) and the engine condition. If the vacuum falls within this range, road test the vehicle.
Note that adjusting the idle mixture screw BEYOND the manufacturers maximum generally will not change the A/F ratio.
Not saying this is the absolute best method; but I have been using this method successfully for over 50 years.
Jon.
