Hey guys so I'm running a super six manifold with a weber carb I also have Clifford headers and electronic ignition so my problem is I get too much vacum out of vacuum advance port at normal idle 750 which makes my car backfire thru manifold and carb because it screws up my vacuum advance what should I do I have it idleing at 500 now which puts zero vacuum on the port and it seems to help except now my idle is mega low anything I can do maybe drill a hole on primary throttle blade? Please help and bye the way carb is freshley rebuilt and valves adjusted on point as well as dizzy is brand new thanks in advance

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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.
Jorge.
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Before drilling holes, there should be a secondary throttle stop adjustment of some kind. Open up a little,get more idle air that way for testing.