Morgan carbtune I am speaking of doesn't use fluid uses metal rods. Maybe you are thinking of the old school mercury ones or the manometers...check it out
http://www.carbtune.comHere is a side by side comparison from an article in a top British bike magazine.
RiDE is the most respected motorcycle accessory product testing magazine in the UK.
In the test RiDE took 3 random samples of the Carbtune II and 3 each from four other manufacturers. Two dial vacuum gauge systems and two electronic carburetor balancing systems. They tested them on 6 different motorbikes with experienced and inexperienced users.
Then they sent them to an independent instrument testing facility.
The Carbtune II got the highest overall score
It was the only carb sync tool to be "Recommended"
It was the most accurate multi-gauge system tested. Have used em all and am biased :-).