Butterfly valve idle position. I set the bottom of the butterflies flat with the bottom of the first progression hole and use threadlocker to fix it so I will not move it by accident or desperation, for the people who have work on DCOEs or IDA/IDFs it will be familiar, the 50 or so RPMs available with the 1/8th of a turn limit either way does not make much of a difference but it could easily mess up in the tuning of the Idle-progression circuit. Now, this is my method and I know there are other methods from people that are more qualified than myself and in no way or form I'm saying that this "is" the holy method, it is what has worked for me after hitting the wall so many times.
Reasons. When the butterfly is set at this point the three progression holes since they are connected to the idle-progression duct and don't have check valves they let air go into it leaning the mixture reaching the idle port, they together with the calibrated bushing work as an air corrector jet/emulsion tube of sorts for the idle-progression circuit. When the butterfly starts to open this first progression hole is exposed to vacuum from the manifold so now instead of letting air in it lets mixture out to keep the air fuel ratio within range, but also since it is no longer supplying air to the mixture going to the idle port this mixture is also richer which is good since the flow from this port has been reduced from the lower vacuum available, so in other words, holes above the butterfly let air in and below the butterfly let mixture out, the process is the same when each of the progression holes becomes exposed to the vacuum below the butterfly, enriching the mixture furthermore since the volume of it does not increase linearly due to the ever lower vacuum acting on them.
At idle, if the butterfly is below the first progression hole and the idle mixture screw is set properly, as soon as you start to open it more air will rush in but no increase in mixture will be available even worse since the flow from the idle port will be reduced due to lower vacuum sending the air-fuel ratio to the roof and a lean spot will appear until it reaches the first progression hole. When the butterfly is above the first progression hole at idle the response of the idle mixture screw will be little to non existent depending on how high it is the butterfly, but things don't end there, since you have two ports flowing mixture into the manifold and an enriched mixture to boot, it will mimic a "too big idle jet", and then when you correct it you just have leaned whatever is left of progression letting you believe that the main circuit needs to come earlier than actually needed
If after the carburettor and the timing are spot on you find yourself in need of more idle speed because of the alternator or power steering, etc. you can drill a 1mm. hole in the butterfly and if still not enough you can keep on increasing the hole 0.5mm at a time until you reach the point you want.
Sorry it has been this long but the devil is in the details. I hope I have covered all the bases.
I had to read this several times. I don't think it's all that sensitive, Heck, when my carb bowl runs dry, I can pour a shot glass of fuel down the carb and the engine will idle just fine until the fuel pump fills up the fuel bowl again.
One thing to remember is that the idle mixture control
screw is more of a flow control than a mixture setting. the air/fuel mixture is set with the jets ports before the idle screw so there is plenty of pre-mixed fuel available for the transfer port. THe mixture in the well well does get richer as the throttle passes the transfer ports but more air is going into the engine also. There also is the accelerator pump that helps the mixture at the times between the main circuit starts to work.
I think that the vacuum level is quite high anytime the throttle plat is near the transfer ports. not quite like you describe it. Even though the manifold vacuum may be reduced a little the air rushing pass the transfer port creates a vacuum on it own that is quite high forcing more mixture out the port so there is no lean point to worry about.
You also have to consider the manifold design. a long manifold on an I6 acts like a plenum chamber that the cylinders can draw off as needed so the mixture control is not as isensitive as on an DCOE delivering fuel to each individual cylinder. We're still talking tractor engines here not a 900cc Fiat Abarth engine.
The hole drilling is a good idea, even my BBD off a V8 has holes in the throttle plate. One thing you have to remember though is that with the hole in the plate you need a richer mixture available in the well but still have the idle mixture control screw in the right position. Other wise if the mixture control flow screw is out to far, flowing a lot more mixture, the not enough mixture will be available for the transfer ports. and a nice little off idle bog. So your hole size should be just enough to get your RPM up to the right speed when the throttle is closed at least below the 1st transfer port, AND the idle mixture control valve is in the correct position.
You will see many people mentioning the correct number of turns the mixture control screw need to be set at.