I have twin weber 40idfs to fit and an 034 svda distributor for ignition. When i set my float heights last night i found the vac advance ports on the carbs have a blanking screw instead of a take off tube.

I can't remember my last twin carb air cooled set up so was wondering do i need to take off both carbs or will one work properly?

What have you done with your same carb/dizzy set up?

Use the vacuum port on cyc barrel #4. It is located here in green:
Also, recommended is an anti pulse valve. It smooths the vac adv. They can be purchased from Aircooled.net. The Bob Tomlison Weber book talks about it.

Click to view attachment

Thanks for that. I was looking at the 2 small screws in red thinking they were my take offs!

Double checked and mine have the take offs circled in green, they have a blank screw and i misread the exploded view diagram at first.

Dellorto's only have one port if the were used on Alfa's and the factory fitted the valve you speak of, not sure if the Webers ever had provision for that area in the casting to be drilled ,this can be done on the Dellorto's however.
I researched this on a UK bus website and to be honest I didn't know of this and always thought you could run off one vacuum point,ran single throat ICT's on my buggy with one pickup and it ran fine.

If you merge the two carbs with a t-fitting before the distributor you can eliminate some if not most of the pulsing.

I ran both to a T and also added an anti-pulse valve that aircooled.net sells

I originally had just a T. John from ACN told me to only use the one side and also to get the anti pulse.

No personal experience but research indicated port vacuum of 2 carbs sorts it if you can do it,the Webers appear to be changed castings over the years so maybe that's an option.
Who knows there a lot on the net any way.