How to make a water manometer

More sensitive than "carb sticks" for under $10

Posted April 2, 2004

If you work with multi-cylinder, multi-carburetor engines for very long, you will want a manometer sooner or later. A manometer is a device for measuring the difference in pressure between two systems.

There are a multitude of commercially-available manometers, the most popular of which are the mercury "carb sticks." Running somewhere behind them are the TwinMax electronic system and a host of others. Each of the commercially available systems has its positive and negative sides, plus they all cost a considerable amount of money.

However, for less than $10 in parts, you can make your own manometer that doesn't take batteries, doesn't contain any toxic mercury, and can never become uncalibrated. Here's what you'll need:

Materials:

• 20-25 feet of 1/4" clear vinyl tubing (about $2)
• around 15 zip-ties (about $2-3)
• 8' length of 1x3 (less than $2)
• several ounces of water

Tools:

• 1/8" drill bit (or whatever size will pass your zip ties)
• 1/4" drill bit
• drill
• wood saw
• scissors
• large syringe (ideal, but not required)

Procedure

Examine the diagram on the right. It shows a completed manometer. Construction is as simple as it appears.

Cut your 1x3 board off at about 5 or 5 1/2 feet long; the exact length is unimportant. Drill 1/8" holes in two sets at lengths along the board. Each set will have a zip tie passed through it, so space them about 1/4" apart to fit the tubing. Space the sets evenly across the width of the board. The total length between the bottom of the loop and the top set of ties should be at least 4 feet. You may want to have more or less zip ties than are illustrated in the diagram. When I made my "nice" water manometer recently, I ended up using 11 zip ties total: 5 down each side, and one at the bottom of the tubing loop.

Once your holes are drilled, poke a zip tie through one of each hole set, so that the head of the tie is on the back of the board. Find the middle of the tubing, and lay it down at the base of the loop. Then, working up the board, loop each zip tie over the tubing in turn, until it's all fastened in place. Use the scissors to trim off the excess tie material.

Don't tighten the zip ties too tight, or you'll collapse the tubing and it won't work right.

You should have 5 or so feet of loose tubing hanging off each side of the manometer when you're done. These tubes will lead to the vacuum ports of each cylinder when you're using it.

Finally, drill a 1/4" hole at the top, so it can be hung from the ceiling, or a nail on the wall, or whatever. A coat hanger makes excellent hook material if you want to make a hook for the top.

Filling it

Filling up this kind of manometer is actually much more difficult than you might imagine. There are any number of ways to go about it, and I'll describe a few of them here.

The goal is to get about 2 feet of water in each "side" of the manometer when you're done.

Immersion method

Before attaching your tubing to the board, fill up a bucket or sink with water, at least several inches deep. Submerge your tubing in the water a bit at a time, filling the tube as you go. Once it's all full, grab each end, and pick up the tube.

Blowing with your mouth, or just lowering one end of the tubing, empty it out until there's only about a four foot length of tubing with water in it. Carry the tubing by the ends back to the board and attach it.

Vacuum pump method

Immerse one end of the tube in water, and attach the other end to a hand operated vacuum pump. Use the pump to fill the tube part way, and even it out until you have about four feet of water in the tube.

Syringe method

Using a large syringe (like a marinade injector from a kitchen store), slowly push water into the tube until it's about four feet full.

Evening things out

Once the manometer is full of about four feet of water, and is attached to the board, the easiest method I've found for getting the water to even out is to let it sit. Eventually, gravity will do its job, and the water will all fall to the bottom of the tube.

If you're feeling impatient (like I usually am), you can bounce the bottom of the board off the ground, forcing the water down.

DO NOT use compressed air or any kind of powered vacuum pump to try and even the water out, you'll most likely just blow out all the water and have to start over again.

Material improvization

There's no requirement that you actually use any of the materials in my list, above. For instance, I had a water manometer for years that was mounted to a piece of 1" PVC pipe and had 3 zip ties holding it down. No holes to drill, and even cheaper than wood and 11 zip ties. I found it to be ugly, though, and eventually reworked it to be mounted on a proper board -- it's also easier to read that way.

You can use just about anything that's around 5 feet long and can have tubing fastened to it. If you've got a bundle of wire lying around, tie on the tubes with wire. If you have a bucket full of epoxy, use that instead of ties. The possibilities are endless. You could even leave the tubing capped, and unmounted, so that you could carry it on the bike with you for mid-desert carb balancing.

You can also improvise the pointer material, if you didn't want to use water. I've seen people use a design like this (but smaller) with motor oil or automatic transmission fluid. Oil or ATF provides more damping, making it easier to see the overall trend of pressure, rather than the instantaneous changes.

Water, being so light and low viscosity, will react instantly to any changes in pressure -- this can be good or bad. I recommend you try this design with water before substituting materials inside the tube.

Water is a very safe material to work with, even around running engines. It won't cause brain damage like mercury will. Should it be sucked into your engine, it will be vaporized, steam cleaning the combustion chamber as it goes. (I've done this on a poorly balanced BMW with no ill effects.) I don't recommend repeatedly sucking water through the same cylinder in one session, since you could over-cool the cylinder.

Using it

In order to use the manometer, just hook each end to a different cylinder's vacuum port. Most modern motorcycles with multiple carburetors will include ports for balancing the carburetors.

With each end attached to a cylinder, the fluid will swing to show the relative pressures in each cylinder. If one cylinder has more vacuum/lower pressure, the fluid will climb that side of the manometer. The goal is usually to even out the fluid, so the level is even between the two sides. Adjust the carburetors via whatever method the manufacturer provides until the fluid is about level.

Note that with water, the manometer is very sensitive, and you probably won't be able to get the levels perfectly even. Just strive for close. I usually consider within 2-3 inches to be very good.

Adjust the balance both at idle, and at some low throttle opening, like 2-4k RPM. Idle balance may be affected by different adjustments than part-throttle balance, so make sure you're changing the right adjuster for the condition you're testing. At large throttle openings, the balance can go way off, but there's no practical way to adjust out that difference, and it's not very important. What's important is that at steady throttle, the balance is good.

The concept

The idea behind a manometer is that you hook it up between two systems (such as two cylinders in an engine), and the pointer material in the middle -- water in this case -- is sucked toward the side with lower pressure. This water manometer works similarly to mercury carb sticks, with two exceptions: the water isn't as dense as mercury, so you need more room for it to move around; a four-tube mercury manometer actually measures each cylinder against atmosphering pressure, not each other.

When you see that one side is higher than the other, you know that the cylinder attached to that side is pulling more vacuum than the other cylinder.

Created by Ian Johnston. Questions? Please mail me at reaper at obairlann dot net.