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MG MGB Technical - Water manometer for carb balancing?

Would it be possible to make a big water manometer to balance twin carbs with? Since water is so much less dense than mercury I imagine it would need to be about 6 feet tall but still that is easy to make from a board, a long length of vinyl tube and some coloured water. Since it is the difference in pressure between (rather than the absolute which would just suck the water into the engine) the two carbs I imagine it would be easy enough to tune by ear to get it close enough then use the manometer to get it nicely balanced. Anyone tried this?

Simon
Simon Jansen

You probably could, but the little SU toolkit that Moss sell would be a lot easier. ;-)

I built a water U-tube manometer to help set up my bagpipes (and to help me learn to blow steadily). It is a handy tool.....
Rob Edwards

Now there is a joke in there somewhere about bagpipes being as hard to tune as SUs explaining the sound they make but that would be mean :)

Does anyone have a link to the Moss toolkit (buggered if I can find it on their site)? Problem is I have to order that specially and pay for shipping to here where as a length of plastic tube and a bit of board I can make up in my garage in no time.

I guess it is a try it and see thing. I just figured someone else may have done it before.



Simon Jansen

It's item 'B' in this illustration from Moss USA. There must be someone closer to you that has it -- SU Midel, maybe?

http://www.mossmotors.com/Graphics/Products/Large/BAC-438.gif
Rob Edwards

I'm sure you could bodge something entertaining. I remember my physics teacher once boasting that he'd made a clever set up with a clear tube and a captive table tennis ball that the carbs of his Rover P6 would fight over, but that was a long time ago and I didn't have a clue what he was on about at the time. Seems to make a little sense now though.
Steve Postins

Simon
There is a gadget, but using it actually affects the airflow.

In the meantime, get a length of small bore plastic tube (say 3mm), stick one end in your ear, and put the other end in the inlet of the carb.

Fiddle untill the 'suck' noise is the same. (I think that the technical term in the 'SU' section in my old TF manual is 'hiss')

Ensure that the tube is placed in the same relative position in each carb as you swap from side to side. I used a few turns of tape around the tube to act as a depth guage,(about 10mm from end) and placed the tube along the bottom of the intake.

You can be certain that the setup will change when you reclamp the butterfly spindles.....

Cheers
Ian F
Ian Fraser

Simon, if you have any friends with multi carb motorcycles they may well have a set of "carb sticks", a four column mercury filled balancer.
John H

What is difference in pressure you are trying to measure at the carbs? 1psi is about 2'5" in old units, say 730mm. It might pay to insert something like a fuel filter in the low pressure side to give the water somewhere to go rather than into the carb - should the pressure differential be higher than expected.
Vic
V Todman

I have a proper medical stethoscope I was using (gaining me fully looks from people walking past my garage). I can definitely hear a difference in noise and was able to adjust to get them the same. I am just not sure how well I am judging the difference.
Simon Jansen

Isn't there an SU contraption like that for testing fuel pumps? It seems I saw a pic of some huge manometer in an SU book somewhere...
Jeff Schlemmer

That was funny looks I mean.
Simon Jansen

Hi Simon

When you can get them sounding the same with the butterfly shaft clamps done up coupled with a smooth idle you are there.

Once you are driving, thre are a whole lot of other influences that will make one carb act different from the other and short of spending hours checking air flows on each carb on a bench and on a dynomometer there aint much you can do about it.

Cheers
ian F



Ian Fraser

The "unisyn" tool with its variable restriction and little red ball that bounces up and down is the standard tool in my experience - and works rather well.

As a practical matter using a tube and adjusting to a common hiss works as noted above.

Some of the carbs have a little lifting button under the piston on one side or the other. When you push the button upward it lifts the piston and reduces the vaccum. There should be a similiar reaction with either carb when things are balanced.

There are two issues (assuming the 3rd factor, cylinder compression is even) 1/ is the throttle butterfly equally adjusted in each carburetor and: 2/ Is the mixture adjusted properly for each carb.

The priority is getting the throttle opening the same, then adjusting/fine tuning the mixture. Leaving the mixture slightly rich, really helps with cold running. The rich idle mixture has virtually no effect on the mixture ratio under load.

I had one engine with a modified cam that idled best with one carb almost closed and most of the idle air coming from the other carb. I think it evened out the back pulses from the modified cam.

Barry

Barry Parkinson

In my experience the SU Toolkit works better than the Unisyn because you can not only synchronise the carbs at idle, but also observe how well they stay in synch at any rpm. You can also compare how they rise and fall together. It also gives a good visual reference for using the lifting pins -- which espcially helps when setting the mixture with the air cleaners on.....
Rob Edwards

OK, I tried a water manometer. I had no idea how much lift I would see so I started with a 1.2 metre long board and tubing held in place with little clips. I just used duct tape to hold the hoses up near the mouth of the carbs at right angles. At first it didn't work but then I found you have to have the tubes right in the carb throats pointing into them. That worked then and I could see about 6mm difference in the water column heights. By changing the throttle screws I could get them to level out. So I think it is working and it seemed fairly sensitive. What I did was cut down the board so now it is much shorter since I only get a small water difference. I used an air filter gasket and marked out a metal plate that will go across the face of both carbs and bolt up to them with short bolts. There are holes where the carb throats are of course. What I shall do is pick up two small right angle T pieces from the auto parts store. The leg of the Ts will be epoxied to the metal plate so one half of the T points into the carb throat and the other end points out. To the out side my manometer tubes will attach. The two Ts will be positioned so they are both in the same place in each carb so I am measuring the pressure in the same place at each. Just playing around with the hoses it seems very important to get them in exactly the same relative positions.
Simon Jansen

You can get the gunsons Carburettor balancer here in Sydney, probably available somewhere in NZ, not the best in tools but usable!
Eric Martin

Simon, are you familiiar with the use of pitot tubes? You may want to Google that. They can give you a much more accurate reading than just a simple tube or T in the airflow, and the position of the hoses will become unimportant.
Jeff Schlemmer

Jeff,
Pitot tube, as in aircraft? Wouldn't you have to have the two tubes going inside the carb and then pointing back out? Or would it also work in the opposite way too, i.e. measure a negative pressure?
Dave Smith

That's just getyting over-complicated. All you need is a tube with one end in the reduced pressure at the carb inlet and the other at atmospheric i.e. further away, and some means in the tube of measuring the difference in pressure i.e. a little plastic gizmo as per the Gunsons or possibly a U-bend with water in it. The Gunsons etc. gizmo measures *flow* rather than pressure, and as such I can imagine it being more sensitive that water in a U-bend, but have never tested it ... since I have a Gunsons.
Paul Hunt 2

I also use my medical stethoscope to balance 'em. Not as accurate as a rolling road and emmissions sensor, to be sure, but if it's good enough for detecting subtle heart murmurs (which I use them for too), then it's more than good enough for balancing my SU's!
Curtis Walker

This thread was discussed between 22/02/2006 and 26/02/2006

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