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MG Midget and Sprite Technical - Vacuum Gauge Position

I have purchased a Smiths vacuum gauge which i would like to fit to my 1275 midget.
Used one on a Renault 4 in the early 70's mainly for tuning but also for keeping an eye on the fuel consumption a handy piece of equipment.
But due to the twin SU's where on the manifold do i locate the adaptor or is there a take off pipe i can use.
Thanks
Stew
s johnson

Is it a 12CE or a 12CC engine? Or better question - do you have the trumpet style Smiths breather valve on your intake manifold? If not then the manifold will have a blanking plug in the top of it. 3/4" BSP I think. You'll need an adaptor down to the small guage pipe but it could connect there. Alternatively put a T piece in the vacuum advance take off and use that. Either way make sure it's all air tight before you start her up again.

I had a vacuum gauge on a kit car I used to have. A most interesting (distracting?) guage indeed.

Ant
Ant Allen

Ive been kicking around having one permenatly mounted in the intake....my uderstanding there are 2 kinds of vac gauges.

ones that have fast moving hands and measure everthing, and auto gauge kind that have a "tolerance" and wont measure every little hick up, thus wont dance like a virgin mary dash cap figurine in a hells angle bikers square dance/clogg off...

Hit that fiddle Tunis


Prop...swing her to the left, swing here to the right...Prop
Prop

I have one and it connects to the inlet manifold. I think the gauges are essentially the same but normally have a small restrictor to dampen out the pulses. Otherwise the needle will do a dance and make reading and tuning the engine almost impossible.
f pollock

With twin Sus, you could do with two, one fitted to each carb. The pipe is usually connected on top of the carb flange where it meets the inlet manifold. The Smiths gauges when new came with a small tube connector that screwed into the carb flange.
It's important that you use the correct size bore of pipe and a restrictor is sometimes required if the gauge fluctuates a lot. If they are not set up correctly, you will get false readings.
Have you got the one with the coloured face, or the black and white type?
Cheers John
JOHN HALL & JULIE ROBERTS

You should measure manifold vacuum, not carb (ported) vacuum...

A
Anthony Cutler

I reckon we're talking about two different things here. The vacuum guages used to set up a pair of carbs will be interested in port (and manifold?) vacuum, but that's for setup and tuning only. I thought we were talking about a dashboard vacuum guage (sometimes called an economy guage but basically it shows engine "load") which needs a single manifold vacuum takeoff.

Ant
Ant Allen

A bit confused for a moment , but like Ant says this will be mounted to the dashbord.
I would think that the take-off will be on the manifold between both carbs to measure manifold vacuum.
John/Julie.It is the coloured segment one but i have an old b&w one with numbers gives a better reading for tuning.
Stew
s johnson

The diagnostic variety are unbuffered (no restrictor) because you WANT the needle to dance around a lot. The dance tells you lots of useful information about the state of your engine.

The buffered variety tell you about how much load is on the engine (the lower the vacuum reading, the more load), therefore how fast you are burning through the dino-juice. Usually, the dash-mount units are buffered.
David "no gauges to measure the vacuum between the ears" Lieb
David Lieb

If you want to turn add a restriction to the signal tube on an un-damped gauge then tie an overhand knot in the tube. The knot can be pulled tighter to change the degree of damping.
robnrrugby

Hey John,

Im looking, but I cant seeing what your saying, but thats how I am...... are you talking about the brass vac ports just above the choke linkage, that hook to the "Y" pipe that goes to the front engine breather cover?? Got a pic, or enginered drawing, I had thought you just hooked a guauge into an open port on the intake manifold...but never saw the logic of that thought.

I am intrested in this method, as the rubber tube in the ear, and down the carb throat, is WAY to scientific for me....LOL

Prop...I've become "Comfortably numb"...Prop

>>>>>>>>>>With twin Sus, you could do with two, one fitted to each carb. The pipe is usually connected on top of the carb flange where it meets the inlet manifold. The Smiths gauges when new came with a small tube connector that screwed into the carb flange.
It's important that you use the correct size bore of pipe and a restrictor is sometimes required if the gauge fluctuates a lot. If they are not set up correctly, you will get false readings.
Have you got the one with the coloured face, or the black and white type?<<<<<<<<<<<<<<
Prop

Carb flange??? do you mean the black plastic/rubber block?

if so, do you mean .... drill a hole in the black block and put a fitting in and use that to hook up to the vac gauges, is that correct???? for both sides???...2 vac gauges???

prop
Prop

do you mean something like this


Prop


Prop

I know what you are saying Ant, but if you think about it all the vacuum gauges measure the same thing, vacuum in the manifold, it doesn't matter where you fit them, on the carb or on the manifold.
I ran twin 1.5" Sus on a cooper in the 60s and had two gauges fitted, really for the fun of it. Motorbikes use a multiple set up for tuning the carbs, If you fit them on the dash, you have a constant monitoring point.Spacer plates with a takeoff tube that fit between the carb and the manifold were available, not easy to find now, but it's an easy job to fit a tube into the existing plates or on the manifold.
Cheers John
JOHN HALL & JULIE ROBERTS

You want to measure manifold, not carb vacuum - there is a difference:

1. Ported carb - the vac pipe take-off is on the air-filter side of the carb at idle... so no vac measured; as the throttle is opened, the butterfly sweeps across the vac take-off so it can 'see' more inlet vac. So you don't want 'ported' vac reading - it doesn't represent manifold vac except at full throttle, but even then...

2. ...when there's an air-flow across the vac take-off (as there will be for 'ported vac' at full throttle or any other situation where the vac take-off is in a venturi), then the effective air pressure at the take-off is even lower (Bernoilli effect). You are therefore measuring manifold vac plus pressure drop due to air movement.

This effect take place in the manifold, too, but to a lesser extent; for this reason, the vac take-off for the twin SU manifold is in the balance pipe and not one of the manifold runners.

HTH

A
Anthony Cutler

This thread was discussed between 07/05/2009 and 09/05/2009

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