DIY Manometer

I am building my own manometer based on some designs floating around here. I have a question on the type of 4 way connector to use. Below is a picture of a simular 4 way splitter that I was able to find. With this type, two up and one out each side, are the results going to be accurate because of the differing amounts of fluid in the shorter top two hoses VS the longer two side hoses? I was looking for two of the "F" style connectors to connect end to end together but have not been able to find them. If the "F" style is better because the volume would be the same for all four tubes, then would two of this style(in the picure) work with the outside ends caped and the center two joined together?

 

It'll be just fine.

Since it's measuring pressure the only thing that matters is the height of the fluid. You could make one 50 feet long, run it down to the basement and back up and it would be just as accurate.

 

How do these home-brewed manometers give you an accurate result without there being an independent reservoir of fluid to draw from.

With all the hoses connected to each-other ... there has got to be some robbing of Peter to pay Paul causing a variant in the height of the fluid under vacuum.

If the Carbs are all pulling the same vacuum ... where's the reservoir that makes the fluid height independent of what's being pulled by the other three?

 

If I understand it correctly, the theory goes as follows: Yes, a change in the vacuum on one cylinder will affect the reading on each other cylinder, and if we were trying to set the vacuum to a particular level (or measure each cylinder's vacuum independently), it would be extremely difficult (if not impossible) using this setup.

However (and this is the trick), we don't care about the vacuum level of any of the individual cylinders. The only thing we really care about (at least in terms of synchronizing) is each cylinder's vacuum relative to the others - more specifically, we want them all the same. With this setup, the only possible way each tube can have the same fluid level is for each cylinder to be pulling the exact same vacuum - so just adjust the carbs until the levels line up. By definition, that means you're synchronized.

The fluid height in any tube isn't independent of the others, but it shouldn't need to be. Let's say that you have a perfectly synchronized rack of carbs, and you put one of these homemade jobs on it. What you'll see on the tubes is that all four fluid levels will be equal. Now let's say you adjust the #1 carb to pull more vacuum. The level in tube #1 will rise, and the other three will fall, but 2, 3, and 4 will still be equal to each other, since they are still pulling the same vacuum relative to each other. Syncing with one of these tools should be just a matter of adjusting until the levels are equal.

I haven't done it yet, so I have no idea if it works in practice, or if there's something I'm missing that makes this type of device not work. Plus it's 12:30 am, and I can't tell if I'm making any sense or not.

Any reports, stories, anecdotes, or whatnot from someone who's actually used them?

 

I'm sorry.
I disagree.

Unless the Fluid Volume is shared from a supply that allows them to draw-up from a reservoir the readings cannot be depended on.

Further, the reservoir should be open to atmosphere ... where Vacuum will allow the level to rise independent of the others.

A closed system will not be accurate. The vacuum will work to collapse the tubing and make the level rise as the tubing shrinks. The vacuum present does not have to be equal to get the device to freeze at some point where the fluid isn't vented to allow it to "Go to" the decrease in pressure.

All that tubing and fittings is just a colorful visual if it cannot pull the fluid because its fighting a vacuum within the device.

Thats why you can invert an open liter of water and not have the water spill out. The vacuum within the closed system prevents the fluid from spilling out of the container.

Just as the vacuum within that closed system prevents the fluid from actually rising to an accurate height being drawn by the Manifold Vacuum.
As soon as the device is fighting itself ... Manifold Vacuum on the vacuum created within the device ... its accuracy is suspect.

The fluid dynamic following the first rule of hydraulics is for the fluid to seek its own level.
A false indication that the carbs are in tune.

Put a reservoir of fluid open to atmosphere on a fifth hose and you'll get an entirely different reading.
Responding to the Vacuum by drawing the fluid higher ... not,
sucking the air out of the lines and making it difficult for the fluid to move.

 

True if looking for an absolute reading, not for relative readings though.

This is the most valid argument I think. Fluid climbing the tube due to surface tension is dependent on the width of the tubing. Glass tubing at the measuring height would eliminate this problem, but my gut instinct is that it's a second order effect and not that significant given how similar the vacuums will be.

Yes it does. (ignoring tube shrinkage and surface tension) The height of fluid above ground exerts pressure down. The vacuum exerts pressure up. The pressures must match if the heights are equal. It IS that simple.

This is true but not relevant to the manometer problem I think.

Is this a YICS issue? I'd agree that the YICS shut off device thingy is necessary otherwise it could be quite unpredictable behaviour.

True in an open system and ignoring surface tension effects. Not relevant here though I don't think, and true because
'height of fluid = pressure above fluid (x a constant)'

True, this will be a more accurate set-up, a small vacuum change but will result in a larger fluid movement. But the closed option will also work.

I used the two arm version last week to balance my carbs and I wouldn't have wanted any more movement than was there.

From an Ockham's razor perspective I'd say stick with a two tube system since it's simpler. It will need a couple of iterations to be sure it's accurate but that doesn't take too long. I can't make rational argument against the four tube system except that it's more complex and therefore less good !

Kevin Hollis ( PhD in physics, although I would probably trust an engineering graduate more on this kind of stuff ! And my bike mechanics is weak! Incidentally you'll see in my new thread that I've spent too long carb cleaning and have moved on. )

 

Rick, you are correct. The piece you are missing though is that they do share a common reservoir. The tubing itself is the reservoir.

Here is a 2 port manometer I drew up quite some time ago:


I think it's pretty clear that for the level to match on both sides the pressures at the two carbs must be equal.

You can think of the bent part of the tubing as the reservoir - although actually the whole tube is the reservoir.

You could connect extra tubes at the bottom for additional carbs and it still works right.

You might be thinking an open reservoir is required. Imagine two tubes with the bottom ends dipped in a bucket of fluid. That's not equivalent to the manometer drawn above. The one with the bucket is a 3 port manometer - 1 port to carb 1, 1 port to carb 2, and 1 port (the bucket itself) to the atmosphere. You ignore the level in the bucket because you don't care about atmospheric pressure, you only care about the pressures in carb 1 and carb 2.

 

So what I am getting out of all of this is that a 5 way splitter with one going into an open bucket, bottle, tub, or whatever, would be the preferred method.

 

Hey ! that's my crappy picture. My first rendition used clear hospital oxygen mask tubing (free) and it didn't collapse under vacuum. It did tend to trap air bubbles. The carb side was $5 clear tube from Home Repo.
So I took it apart and put the larger diameter tube on the fluid side, and bubbles work themselves out quickly, and there's no capillary attraction on 1/4 inch tube.

You could build an open system. Put your bike on the third floor of a parking garage, a bucket at ground level, and an assistant on the second floor calling out the levels, because that is the absolute vacuum levels using water (and oil is higher still) 33 feet = 30 inches of mercury.
At idle, the levels would be 10 - 15 feet, and a throttle blip could pull to 25 feet.

I've used mine 5 times already and I'm happy with it.
(in case it wasn't clear, we're not building 30 foot manometers; closed system 4 foot vertical, then filter-restrictors taped to your mirror, then down to the carbs)

 

I tried building a 4-tube manometer to tackle all the carbs at once. Didn't work to well. I wasn't able to tame the readings before one would get to the top (and on its way to the engine). I tore it apart and went with the two tube setup, and it worked out much better. I suppose if I started with the two tube setup, then fine tuned with the four tube setup, that might be the best route.

Anyway, thats how my tune went, at any rate. She's running now, though.

 

TIMEtoRIDE, your "crappy picture" is a lot better then my "no picture"...

So what you are saying is to put the bike on top of my barn and have the bucket on the ground... Ok, I think I will just go with the closed system for now, get it synced close and then maybe do plug chopping to really get it good.

 

My system cost $11. You must make restrictors to tame the pulses.
I bought 4 inline fuel filters, and epoxied 1 inch of the red spray tube from WD-40 into the carb side of the filter. My fluid levels are quite steady.
I haven't tried oil; water is a little heavier.

If your carbs are WAY out, one tube will suck to the top.Just keep your finger on the kill button, and tweak while the engine isn't running, try again. I sucked water into the filter a few times, but only a little bit ever made it into my engine.

 

my redneck setup is like what is pictured in MiCarl's post. My friend has a steel pol in the middle of his garage for support reasons. We took 25' or so of tubing started 1' from the floor made sure we had the center of the tubing as the bend filled it then ran the tubing straight up the pole. the ends for the carbs(with restrictors built in) come down from overhead and are awesome and out of the way whle tuning. We took a yardstick and taped it and the tubing to the pole for accurate measurements. Works nice and is out of the way when not in use. all you have to do is pull up near the post with your bike, sled whatever you got and sync away.

 

Totally unnecessary. You only need an open tube or bucket if you want to measure the vacuum relative to atmospheric pressure. No reason to do that.