I’ve set the timing on my MGB using the vacuum gauge to great effect but not used it to adjust the fuel mixture …..yet.
I have read that there are several good reasons why vacuum gauge tuning of engines is better than simply – setting say the timing – to the manufacturers standard settings.
1. The standard settings that were made with leaded fuel as it was 40+ years ago! …… compared to today’s unleaded fuel thats burns slower and with additives like bio, ethanol and other extras burns very differently to the old leaded fuel
2. The original manufacturers settings were a compromise between getting the best performance across a wide range of manufacturing tolerances. Machining tolerances were nowhere near as tight and accurate as engine parts are made today. So standard settings were detuned to suit the “worst case” made engine based on minimising the risk of any warranty claims.
3. Tuning your engine with a vacuum gauge takes into account that most classics are higher mileage engines and have a certain amount of wear (poss a lot of wear). The vacuum gauge will tune the engine to the best it is capable of in its current condition – standard settings set the engine assuming it is still new!
4. About 80% of MGBs were sold into the USA where emissions were tightly controlled so original manufacturers settings erred towards ensuring emissions requirements were met rather than achieving best performance.
The link below takes you to a very informative article on how to tune your engine – the ignition timing and fuel mixture to get the best performance (and efficiency) from your engine – in its current state – using a vacuum gauge.