poor bit of arithmetic, mercury goes all dirty, carb manometers.

[graffian]

I have had a little bottle of mercury for about 50 years, "probably enough to fill a set of gauges" said the bloke I scrounged it from.
I need a pair of gauges, to balance my bikes carbs.
But the bottle of mercury was all grey and crappy looking and the mercury in the bottle looked the same as the outside of the bottle.
I estimated I had 3cc and that wasn't enough to make a manometer with the tube I have , so I stuck the bottle back on the shelf.
I have just pulled the mercury up with a syringe to see exactly how much I have, turns out my estimate of 3.x cc was about half!
So I have a nice n shiny 7cc of it.
I haven't got a 4 cyl bike, but I would like another GSX11. So don't really need a 4 column manometer, but maybe I should be looking for a 4cyl set
that need mercury. I wonder how much it takes to fill them? Anyone know about them, cos the ones I used to use were short I am guessing they are
closed one end? if I remember rightly 30inchs of vacuum was about 10 inches on the gauge.

 

[frank horton]

aren't there mercury free gauges.....?
I was tought to do it by ear with a rubber tube in the good old days....lol...

 

[Ross365]

I've not encountered a mercury-filled carb balancer before. But wasn't the idea to observe and tune out the pressure difference between two cylinders, rather than measure the absolute pressure on them? Realise it would have to be the latter for 4 cylinders. I seem to recall 760mm as the height of mercury supported by a good vacuum.

I cannot find a picture of the old mercury balancer anywhere on Google.

 

[MetalMonkey]

You can buy a set of gauges so cheap I wouldn't have thought it was really worth the effort of faffing about with mercury?

 

[Misterg]

if I remember rightly 30inchs of vacuum was about 10 inches on the gauge

Sounds about right.

The amount of mercury you need depends on the bore of the gauge tubes - 1mm bore needs 0.25cc to fill a 1m length.

Not sure that it’s worth the hassle though.

 

[vx490]

I have a book, Four Strokes Tuning in Theory and Practice by A. Graham Bell (1st edition, £6.84 on Amazon, well worth buying). It tells you how to make a glass tube manometer for a four carb setup. He says to use four 6 foot lengths of 5/32” i.d. glass tube, heated and bent into a U shape around a 2” tube. You need 10cc of mercury for each tube. You fix them at the same level across a black board, leave one end open to atmosphere and run a 9 foot length of rubber tube to your manifold adapter. The long rubber tube provides a damping effect. Now given that you have much less mercury than this you would need much smaller diameter glass tubes, and I don’t know whether this would work. He also greatly emphasises the danger of mercury, which constantly gives off toxic gas, and accumulates in the body. Hope that helps.

 

[Seadog]

I had a four tube mercury manometer for carb balancing on my ZX10 c19990.
Of course, now you're a terrorist if you have any mercury

 

[Ross365]

Bending small diameter glass tube in to a U shape might be a bit tricky... unless you are a skilled glassblower. There are types of clear plastic tube (PFA) that you can now get with O.D. as small as 3mm that could probably be used in place of the glass; easier to construct and break-proof too.

Mercury gives off it's own vapour, but I recall having beakers full of the stuff on an open bench at school in the 1960's. However, it is now totally "untouchable". It is toxic, and also, you can make an explosive compound from it, which was commonly used in warning detonators on the railways in the past https://en.wikipedia.org/wiki/Mercury(II)_fulminate.

 

[Nomad]

I love the idea of DIY solutions but I bought a 4 way set of Carbtune gauges many years ago and they have been invaluable, mainly used on 4 cylinder Jap bikes, 600cc and above.

The price has gone up a lot since I got mine, now £85 before £5 discount but good used set could be a cheap option .. https://www.ebay.co.uk/itm/23570275...ttor+Vacuum+Balancer+4+Guage&epid=24025085056

For DIY you have 2 designs, "Absolute" ( a separate gauge for each cylinder - one end of the gauge vented to atmospheric) or "Differential" (ends of the gauge connected to different cylinders to show the difference between them).

For any DIY build, consider adding restrictors (small bore jets or just clamps on the tubes) to dampen the readings - it makes it a lot easier to read them.

 

[Pigeon_Droppings2]

I seem to recall 760mm as the height of mercury supported by a good vacuum.

A younger version of me could probably have calculated it....this version is too lazy

Should "just" be something like ...balancing the weight of mercury vs the pressure difference between the vacuum end and the end exposed to atmospheric pressure.

Edit....just googled it and @Ross365 was right ...."One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury".