How to make a water manometer

(by Ian Johnston - reaper@obairlann.net)

Why do I need one

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

There are a multitude of commercially-available manometers, the most popular of which are the mercury "carb sticks", somewhere behind them are the TwinMax electronic system followed by 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.

On the other hand you can make your own inexpensive manometer that doesn't need batteries, contain any toxic mercury, and it never becomes uncalibrated.

How does it work

The idea behind a manometer is that you hook it up between two systems (such as the two cylinders on your engine), and the pointer material in the middle - water in this case - is sucked toward the side with lower pressure. When you see that one side is higher than the other, you know that the cylinder attached to that side has a greater vacuum than the other cylinder.

This water manometer works exactly the same as mercury carb sticks, except that the water isn't as heavy as mercury, so you need more room for it to move. It also means it is more sensitive than carb sticks.

What do I need

Materials:

20-25 feet of ¼" clear vinyl tubing
zip-ties (approx. 9 - 15)
5½' length of 1" x 3" wood
water (food dye - optional)

Tools:

drill bit lage enough to suit your zip-ties
1/4" drill bit
drill
wood saw
scissors or wire-cutting pliers
large syringe (optional)

How do I make it

Examine the diagram above showing a completed manometer (hover over it to see it in action).

Construction is as simple as it appears:

Cut your piece of wood to a length of approx. 5 to 5½ feet.
Drill holes at even lengths along the board. Arrange these in two sets spaced approx. ¼", as each set will have a zip tie passed through it to fix the tubing to 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.
Poke a zip tie through one hole of each 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. Loop the bottom zip-tie round the tubing and back through the second hole in the piece of wood then fasten the zip-tie at back of the board. Be sure not to tighten the zip tie too much, so that the tubing doesn;t collapse and still lets the water through.
Repeat step 4 all the way up the board. 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 tidy the zip-ties by cutting the excess lengths of the zip-ties using scissors or wire-cutter.
Drill a ¼" hole near the top of the board, or fix a screw hook or loop at the top, so it can be hung up. (from the ceiling or a nail on the wall etc.)
Fill the hosing with water so that you have approx. 2 feet of water in each side. You can achieve this by:
- Immerse the whole tubing in water a little bit at a time filling it up as you go. You'll need to do this before you attach the tubing to the board.
  Then let out enough water until you have only about 4 foot in the tube and attach the tube as discribed above.
- Immerse one end of the tubing in a container of water and suck (either with your mouth or a hand pump) water into the tubing until it reaches the above described level.
- Use a large syringe (like a marinade injector from a kitchen supplier) and slowly pump water into the tube.
Let the water even out. Once the manometer hold about feet of water (2 on each side) and is attached to the board, the easiest method 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 on 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.

Can I change the design

There's no requirement that you actually use any of the materials in my list, above.

All you really need is something that's around 5 feet long and can have tubing fastened to it and something to fix the tubing 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 get the idea.

You can also change 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 very light with 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.