The procedure is to
The procedure is to merely adjust the carb air flows, or more precisely, the intake manifold vacuum for each cylinder, until the mercury columns are equal in height. This is obviously done by adjusting one of the air balance screws to equalize vacuum within a carb and adjusting the linkages as necessary to obtain equal vacuum between carbs.
Note that vacuum decreases for a cylinder when the airflow increases and conversely vacuum increases as air flow is decreases. This opposite to the effect one would see with an air flow meter measuring air entering a carb.
One thing you will notice is when you increase the airflow to a relatively high vacuum cylinder, the manometer column in that cylinder will droop, yet vacuum in the other cylinders will increase slightly as the engine speeds up. This is an effect you normally would not notice with an air flow meter that is not as sensitive and is only looking at one cylinder at a time. The effect of an adjustment can be evaluated immediately. This procedure works especially well when checking off idle balance, say up to 2500 rpm. It's quite easy to observe the relative positions of the multiple columns as they dynamically change.
There is something you have to be careful about though. It's not a good idea to rev the engine and suddenly close the throttle. Under this very high vacuum, long duration condition, there is a danger some of the mercury could be sucked into the intake manifold. The manufacturer claims that a small ingestion of mercury will not harm the motor but it's pretty toxic stuff and expensive to replace.
I removed the hard plumbing on my 308 that evacuates fuel vapor from the charcoal cansiter and replaced it with soft, silicone vacuum tubing for heat resistance and hopefully long life. The remaining holes in the intake manifolds were tapped with a 1/8 NPT thread to accept my barbs of choice. Measuring vacuum here has proven to be a near ideal solution because the orifice connection to the tapped hole to the interior of the intake manifold is quite small in diameter. This small diameter orifice helps to dampen the vacuum signal. It may also be cosmetically reversable.
I'm not sure I'd tap the carbs to obtain a vacuum signal but theoretically, any port down stream of the throttle plate could work. I use a similar method on my S2 Elan with a pair of DCOE's, with excellent results.
The manometer system I selected is the Carb Tuner Deluxe made by MotionPro. It has a plastic high visibility body that won't corrode. They also make a less expensive model, the Economy Carb Tuner. Their online link is:
I think they will sell direct but I bought mine by comparison shopping online by searching on "carb +tuner +deluxe". A good motorcycle shop may stock it or should be able to order it for you. Prices vary a good bit.