Vacuum gauge

 The vacuum gauge is an immensely valuable tuning and maintenance device, especially for old school engines like the MGB. It can be used for anything from ignition timing, carburettor tuning, engine fault diagnosis and even fuel economy if that's your thing. Learning to read and understand the vacuum gauge is the important part of owning one but there's plenty of tutorials online to show you how to understand what your vacuum gauge is telling you. Most people just buy an external gauge and hook it up to do maintenance and tuning. It's easy to set up on the MGB manifold which has plugs for deices not usually used in your car. Ideally on a twin carb engine, especially one with Siamese ports you want the vacuum gauge to run to a T piece and then to each inlet for each carb. This will give you a more accurate reading than running it off the middle of the balance tube on the manifold but once you get accustomed to the readings from the balance tube, it works just fine. 

I run an old period Redex vacuum gauge all the time in my car as it gives an instant notification if somethings amiss, like an oil pressure gauge. Watching a vacuum gauge under all engine scenarios you can see if your engine is healthy or needs attention without any other diagnosis. If you don't care about period correct parts you can pick up a dashboard vacuum gauge very cheap (from $10) or if you'd prefer not to have one constantly on there are cheap ones available online starting at about $20 that you just plug in when you need it. A standard engine will react to a vacuum gauge differently to a modified one, so you need to get used to different settings if you change your engines characteristics, but when you make changes those changes in the gauge make sense. For instance, a longer duration camshaft will have less and a more erratic vacuum at idle. Like I say, you need to learn and understand what the gauge is telling you but once you do, it's one of the best tools out there for maintaining and tuning your engine.