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I have never experienced this before, the car is a 1256 non servo. If I have the idle nearly stalling the dizzy vacuum advance retards how it should but the engine is nearly cutting out or does cut out. If I speed the idle up so it has a nice tickover the vacuum advance on the dizzy closes and advances. I have tried a new vacuum advance which is the same. The internals of the dizzy are new.
Engine is fitted with a Weber carb, the correct spec from Weber for the engine who also say has nothing to do with any excessive vacuum.
I have never had too much vacuum with an engine before, timing is set 9 degrees.

The increase in vacuum is caused by the throttle being almost closed or at least closed too much. If you bring the tick over up a tiny bit by opening the throttle and retard the timing a tiny bit to bring the idle speed down again it should work out OK.

Remember to set your timing with a light at around 900 rpm with no vacuum advance connected. You may also have to adjust the mixture a bit too. Some engines seem happier at 6-8 deg instead of 9 deg.

I will give it a go, I did strobe the timing with the vacuum pipe plugged. Really strange though.

I have set the timing at 4 1/2 degrees and it's still the same. To get a decent idle the dizzy vac advance closes. Is it something that should happen? and I have never noticed before?
It was something that caught my eye after the rebuild and I was checking the engine over.

At tick over there will be a lot of vacuum, the throttle is closed but the engine is trying to suck air from wherever it can so yes, the vacuum advance will close up. A slightly wider throttle will give you less vacuum. Once you open the throttle slightly the vacuum decreases and the advance unit opens up. This advance at idle is what gives you a more stable engine at idle speed.

When swapping carbs over for an aftermarket carb with individual jets like the Weber you may need to play a bit with the throttle opening setting and ignition timing. The trick is to get the best compromise between stable sensible speed cold idle, progression between the idle and main jets and a stable sensible speed warm idle. This is usually not necessary with the standard set up ad the manufacturer has done all this work in testing before production and published the recommended settings.

Generally advancing the ignition a couple of degrees at idle speeds the engine up and retarding slows it down.

The ignition timing is set with the vacuum pipe disconnected and the idle speed low enough that the centrifugal advance is not operating.