Le dernier paragraphe à lui seu est très intéressant...
For a start, there is no difference in running between using carb vacuum to inlet manifold vacuum. The only difference between the two is that carb vacuum gives zero additional advance at idle whereas manifold vacuum gives near maximum advance. Once you start to open the throttle the carb vacuum rapidly rises to meet the manifold vacuum level, and thereafter they are the same. UK cars changed from carb source to manifold source in September 76 but kept the same distributor.
The last time I looked the 123 for the MGB has all the original curves, so you simply select the one you want. The UK spec MGB had the same curve from the start of rubber bumper to the end of production, and only four before that. The big question is how relevant the original curves in the 123 are with today's fuels, and my experience is 'not very'. So much so that at least one vendor recommends the generic 123 over the MGB version on the basis that the curves are more suited to today's fuels. With sixteen switch positions, and a programmable I understand, you can experiment to your heart's content.
http://www.mgownersclub.co.uk/forum/mgb ... c-ignition ( members only )
Never heard that the vacuum signal causes problems before, if the 123 has a problem with it than that is another demerit against it.
As for how beneficial the 123 is, if you replace a knackered or incorrect distributor with a 123 then you will probably see an improvement. But then so would a correct-spec conventional distributor. I also understand from a supplier of the 123 that the curves don't really match up to what you think they are, and you need a session on a rolling road to get the best out of it. All in all far too much buck for the bang you are likely to get out of an MGB - a standard one at least. I'm staying with points while I can, reluctantly changing to an under-cap trigger if I eventually can't get decent points anymore.