Another argument for using Oil rather than Mercury.
I use a "U" tube manometer about 30 inches high half filled with engine Oil, I knocked this up for less than $10 many years ago.
I can really only measure about 20 inches of difference before I'm afraid of sucking the oil into the engine, this is plenty for the purpose of aligning the carbs.
If I was to use mercury, what height of column would be equivalent to my 30" "U" tube
Using my working range of 20" differential.
20 / 13.6 (SG of mercury) = 1.4" So I only need a mercury "U" tube that will measure 1.4" to replace my Oil column
But wait, my oil has an SG of ~ 0.8
(20 / 13.6) x 0.8 = 1.17" Hg so I really only need a mercury column just over an inch tall to balance my carbs.
However I would never be able to keep the mercury contained, if say one of the hoses were to drop off, the imbalance of pressure would blow the mercury into the engine quicker than you could blink.
This is the reason they make the mercury columns tall enough to withstand the strongest vacuum they can possibly see, full manifold Vacuum, As they measure gauge pressure rather than differential pressure they need to be about 30" tall. They also add restrictors in the hoses to damp the action.
Why do we want to measure Gauge pressure? mainly for ease of use with a multi cylinder engine, for a 2 cylinder we just need to know the difference (differential pressure). To measure differential pressure you don't need such a wide range.
My 30" Oil filled "U" tube will not withstand a hose dropping of either but because of the oil viscosity (damping) I don't need restrictors, I have plenty of time to shut down the engine if something goes amiss.
I like the idea of the beer bottle balance tool, I think it is nice and compact, mounted on a block of wood its self standing whereas my column needs to lean against the bike and it's only visible from one direction.
