Home made manometer

[malcpowe]

Hello friends, I have used for many years a Morgan carbtune mercury manometer. I would now like to dispose of this device (youngsters in the workshop) and make an oil filled version. The morgan is only about two feet long, the mercury in the tubes moves at most a couple of inches even at start up. My question is this, how much does the fluid in an oil/water based manometer move - is it really necessary to make one four feet long ?

 

Best wishes,

 

Malcolm R.Powell R1150RT.

 

Shropshire UK

[Ken H.]

It depends on the viscosity of the fluid you are using. Some people use automatic transmission fluid, moves less, some colored water, moves more.

 

I've seen 6 - 8 - 10" of difference (in the water level in the manometer) from one side to the other upon initial startup of a badly out of tune R motor. So in that sense to keep from just sucking water into the engine on the high side, yes a long loop is good.

[Pilgrim]

Yes, and more.

 

First, we can dispense with the oil idea; it's not necessary to use oil, and IMHO, it's a needless complication that answers no questions.

 

Water works just fine. It's available anywhere, and harmless if sucked into the engine. Not that oil will hurt anything either, but it makes a mess, fouls the plugs, and creates clouds of smoke when sucked in.

 

You'll need at least 12 feet of clear plastic tube, fifteen is better. Only about four feet of it actually has water in it, though.

 

In simplest form, you spit a little water into the tubing and let it run to about the midpoint. Slip the ends of the tube over the vacuum nipples on the throttle bodies, and drape the tubing over a handgrip so that a hanging loop is created, with the water in the loop. There should be about 12 to 18 inches of water in each leg of the tubing. More is all right, but less probably won't work very well. Tap the tubing with a fingernail or whatever to work the bubbles out of the water.

 

Start the engine. The water will almost certainly be sucked into the engine, but it does no harm. The engine will cough and sputter and perhaps die. No matter, and no harm done.

 

If for some reason it doesn't suck the water into the engine, if the water stays in the tube with the engine running, you will see that the water column in the tube is jumping back and forth (actually, up and down) so much that it is useless as a tuning tool.

 

 

You just discovered the need for vacuum restrictors. The undamped vaccum signal from the TB is too strong. You must control it.

 

The fanciest restrictor is the core of a piece of coaxial cable such as is used for your television. I speak of the plastic core beneath the insulation, through which runs the copper strand. Strip the insulation from a piece an inch long or so, then pull out the copper core. This creates a miniscule tube. Cut it in half and put a piece in each end of your plastic tube after you've put in more water. You'll find that it is a nice fit, but not so tight that you can't shake it out of the tubing when you need to. (alternate suggestion below)

 

Hook the device up to the TBs again, and drape the loop over the right handgrip, so that it dangles where you can see it easily as you make your balancing adjustment to the right TB.

 

Pinch the tube shut (like you would a garden hose) before you start the engine. This avoids a major excursion from the water during the first couple piston strokes, before they come fast enough one after the other to pull the water back and forth quickly.

 

Once the engine is running, slowly release the pinch in the tube.

 

What you'll see then is the water sort of jiggling up and down in the tube, moving back and forth a small amount in response to each piston stroke.

 

Now adjust the TBs so that the water stands to an equal height in each leg of the tubing.

 

Unlike your mercury manometer, in which the vacuum from each TB lifts its own column of mercury, and has you compare the relative heights of the two independent columns, this device is driven by the imbalance between them. If one TB is sucking harder than the other the water will move that way.

 

There are a few tricks that make using it easier.

 

Some people staple the tubing to a board. If that makes you happy, do it. I don't bother because it makes it hard to throw the tubing in my saddlebags to take on a trip.

 

Instead, I use zip ties to hold the two legs of tubing together, leaving a few feet loose to connect to the TBs. Leave one end longer than the other to reach over the bike to the TB on the off side.

 

Fasten a couple colored zip ties around one leg of the tubing and always attach that leg to the right-side TB. That gives you a quick reference to which leg you're working with when making the adjustments.

 

 

At least drape the tubing over the right handgrip. My tubing, though, is 20 feet long and, when in my shop I hang it over a hook in the ceiling. That makes it more difficult for the water to be sucked out - that's not really much of a factor with the vacuum restrictors in, but I do it anyway.

 

Vacuum restrictors - Above, I described the elegant solution. The first time I made a manometer I just folded a paper match in half and shoved it into the tube. It didn't dampen enough, so I did the same to the other end, and it worked fine. Anything that will restrict the power of the vacuum will do, but if you put something in each end, make sure they are pretty close to the same thing so you'll get equal restriction. I haven't tried it, but in theory a single restrictor ought to work just fine. I use one in each end anyway.

 

It doesn't matter if the loop in the tubing is laying on the floor as long as it is full of water, and water is standing up at least a foot or so in each leg.

 

Colored water is easier to see. If your light is so dim or your eyes so bad that you can't see clear water in the tube, color it. If you use food coloring, a single drop will do, placed into the tube before you spit the water in. You want to avoid putting it in the water first, then taking a sip to spit into the tube unless you just like red (or blue, or green, or whatever) lips. If you're really desperate, remember this: you always have a source of colored water available, but only one selection for color: yellow.

 

It usually takes a little experimentation to find out how it works best for you, but that's no big deal. It is marvelously simple, cheap, and accurate. It's one of those things that's so good I wish I'd thought of it.

 

Good luck.

 

Pilgrim

[ShovelStrokeEd]

Not so much viscosity as density is the determining factor in the length of the tube. Mercury is roughly 13X as dense as water which is more dense than ATF or gauge oil. Therefore the needed extra tube length.

 

You don't want to use the Carb Tune design here which is a common reservoir style but rather a U-tube connected in differential mode. That is one leg to the left throttle body and one leg to the right, this way the manometer will only show the difference between them, which could be a foot of water, rather than the absolute vacuum value. Adjustment then proceeds normally. The taller column has higher vacuum and therefore needs more air to bring it it line (back out the BBS or lengthen the cable slightly.)

[Ken H.]

Not so much viscosity as density

Of course you are right. I get a F in physics today!

[Ken H.]

you always have a source of colored water available, but only one selection for color: yellow.

Yuck! Using this option you forgot one step - THROW THE TUBING AWAY when done!

 

Great write up though!

[bigboy]

please excuse my ignorance , but why noy just use a set of vacuum gauges would this not be the easiest system to use?

If not can you tell me why.

many thanks

derek

[Ken H.]

why not just use a set of vacuum gauges

Oh you could. If you are using a set of high quality gages that are calibrated to each other. Nothing you're likely to find for the $4 it cost to build a manometer.

[Pilgrim]

why not just use a set of vacuum gauges

Oh you could. If you are using a set of high quality gages that are calibrated to each other. Nothing you're likely to find for the $4 it cost to build a manometer.

 

This is one of those stone-axe simple things that works at least as well as high-quality, purpose-built tools that cost 20 times as much. The thing is so cheap that you can buy the material at any hardware store, use it once, then throw it away afterwards, like you'd probably do if you were in the wind and needed a quickie tuneup.

 

I was very skeptical about the concept at first, but after having made one and using it for a while, I'm a believer. No amount you can spend will give you a measurably better result.

Pilgrim

[Clive Liddell]

Ken, Derek,

Although I use mercury columns I also have a nice pair of glycerine filled vaccuum guages for on the road checking. I nearly went crazy with many return trips to the shop exchanging guages to try and get the best match and they are still slightly "out" between them...

[RFW]

It depends on the viscosity of the fluid you are using. Some people use automatic transmission fluid, moves less, some colored water, moves more.

It has nothing at all to do with viscosity! A manoneter's level difference is related ONLY to the weight-per-unit-volume of the liquid filling it.

 

That is why a vacuum will draw about 29" of mercury, and almost 30 feet of water. The difference is caused ONLY by the specific gravity difference between the 2 fluids.

 

Using any kind of oil is pointless because oil has a specific gravity even less than water (about 0.8 in fact), so a vacuum will cause an oil column to rise even furhter than water.... around 37 feet.

 

I just bought a couple of vacuum gauges and calibrated them to read exactly the same (the absolute value of the vaccuum reading is irrelivant for balencing purposes). Then in the vacuum line I jammed a 2" piece of threaded rod. The air passageway becomes a narrow helix (along the threads), so this damps the intake pulses and gives a steady average reading. Without damping, the vacuum needles flail away like crazy!

 

Bob.

[ShovelStrokeEd]

Clive, I too used to use mercury columns and I'm trained in its handling and disposal. I have since stopped. The stuff is just too dangerous. I have two manometers from Dwyer Instruments, actually I have 3 but one is too sensitive being a 1:10 incline. Useful for 0.01" differentials but no bike I have found is even adjustable in that range.

 

I use a 36" slack tube most of the time but I do have a precision bore glass one mounted on the wall near my work bench. Trouble is there are usually 4 bikes between me and the work bench so the slack tube gets the duty.

 

I have never found a pair of vacuum gauges that will come close to the accuracy of a good water manometer used in differential mode. If you must have the portability, look to that same company for a Magnehelic gauge. They come with a center needle and in inches WC I think 15-0-15 which is adequate to the task.

[RFW]

I have never found a pair of vacuum gauges that will come close to the accuracy of a good water manometer used in differential mode.

 

They are if you caibrate them against each other. Simply take the cover and case off one of them, then connect both (via a "T" connection) to the SAME cylinder, and tweak the link in the mechanism of the one that is open to exactly match the reading of the other.

 

The two gauges will now track each other extremely well within the range of expected vaccuum the bike generates at idle.

 

But since a vaccuum gauge has no internal damping (unlike the mass of a mercury column provides), you still need a restriction in the line (see my previous post) to stop the needle from flailing around.

 

Bob.

[malcpowe]

Thank you all so much for your input.

 

Malcolm.