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MG MGB Technical - Fitting vacuum gauge?

I was quite lucky last weekend in the fact that I managed to pick up three smiths gauges for the incredible price of £10 for the lot. They included a vacuum gauge, battery condition and a clock (didn't have one, think they come in handy!)

I've done some reading and I'm almost set on how to wire them in, however, am I right in that the vacuum gauge should connect to the carb somewhere? I have SU carbs and from what I have seen, I need a t connector with a push on connnecter? Anyone else running a vacuum gauge?

The battery condition/volt meter, is this a simple case of wiring it into some of the loom somewhere?

Forgive my inexperience! I will have learnt a lot by the end of this summer after I have put so much time into it, first classic car @ 20years old as a project during my time off!
M J Kirkham

You want the vacuum gauge hooked to the intake manifold. A trip to the plumbing/hardware store should scare up the proper fittings. If you tie into the carbs, the gauge may read zero until the throttle is opened.
Is this the gauge that also reads fuel economy? Higher vacuum normally means better gas mileage.
Jeff Schlemmer

Jeff,

Thanks for your reply. It's just the standard smiths gauge, not the fancy one with the fuel economy on, unfortunately, but same conecpt nonetheless, except it relies on knowledge of the vacuum rather than being told your fuel economy! :)

I'll take a trip tomorrow and find something suitable. Just going to have a look now, see if I can find some info on the intake manifold. Is it a simple job for a newbie like myself?
M J Kirkham

The guage is pretty useless on an MGB. It will show hi vacuum at idle, moderate at cruise and very little while doing any acceleration or grade.

The dual carbs are very large for the car and a little throttle opening lets in plenty of air. You've probably noticed at low and mid range, that opening the throttle more doesn't do much more than part throttle.

The engine doesn't need the carb capacity until the higher rpms.

There are diagnostic uses for a vacuum guage, but a dash guage, typically will not have the sensitivity to be usefull for diagnostics.

Barry
Barry Parkinson

Hi Barry,

I thought as much, it's purely for aesthetic reasons really, just an extra gauge to fill a whole which has been cut!

Cheers,

Mike.
M J Kirkham

p.s. does anyone know how I would wire up the other two? Clock seems simple I think. It says transistor on the face and has two wires, guess it just needs some power, always on presumably.

The battery condition/volt meter i'm less sure on. The B GT is a 1972 UK spec.
M J Kirkham

The clock and battery condition should both be tied into the brown circuit, but fused. I believe the second wire is merely a ground (black circuit.) I, too, once had the vac gauge merely to fill a hole. Its easy to later replace with an oil temp gauge, tranny temp, fuel pressure (with remote sender) or whatever.
Jeff Schlemmer

The volt meter just needs a connection to something hot. Probably a good idea to make sure that the hot supply is switched with the ignition key. I don't know how much draw there is on volt meter but I don't think you want it on all the time.

I have one in mine mounted in the console - it's probably one of the most useful gauges I have. You will see things happening with the voltage that you probably won't like and it will start you off on a campaign to clean up your electrical system to minimize the excessive current draw that most of our old cars have.
Richard Smith

Cheers both. Yes, I can see these gauges making my life harder, the fact that I will be diagnosing problems I never knew were initially there!

I too thought that it might be an idea to have the battery condition switched.

Guess I should study those wiring diagrams again. If only I could find some colour ones it would be easier to understand, rather than using my haynes manual.
M J Kirkham

That's very true - you should have the battery gauge hooked to the white/green wire under the dash. It may draw enough amperage to kill the battery over several weeks of storage.
Jeff Schlemmer

My 67 does not have an accessory position to the ignition switch. If I want the radio on with the engine off I have to turn the ignition system on.

I had an extra flip switch hole on the dash, so I turned that into my accessory switch. The radio, the voltmeter and the aux light plug/cigarette lighter are all controled by the flip switch. The switch is hot all the time - ignition on or off.

Works great

Barry
Barry Parkinson

Excellent. A simple job I admit, but I have cracked it. Got the battery condition running off a switched 12v and the clock running off always live :D
M J Kirkham

Hi.

I "tee'd" my vacuum gauge into the vacuum advance take off on the manifold (as close to the manifold as possible). Another option is to tee it into the brake servo vacuum pipe if you have one.

My Smiths battery condition meter (ie voltmeter) draws 0.1A, so it should definitely be fed from an ignition switched supply.

Don't forget the instrument illumination, which should be fed from a red / white wire.

Don
Don

Don,

Yo're doing well at giving me a hand today aren't you!? Red/white, good, I wondered why I had a spare wire behind my dash just waiting to be wired into them :)

Just studying the haynes manual now as I'm not sure where I should be looking, is it possible to see where on the manifold easily or do I need a ramp or something?

Taa.
M J Kirkham

MJ, give me an email address and I'll send you color wiring diagrams.
Steve Simmons

Steve,

Thanks, my email is michael@agdk.co.uk

Most grateful,

Mike.
M J Kirkham

Jeff,

It's been a very long time since I installed my voltmeter but white/green sounds vaguely familiar...
Richard Smith

Any vacuum gauge is a vacuum gauge and can be used for diagnostics as well. It is very useful on the B and apart from all the usual basic checks is ideal for tuning the twin carbs in conjunction with an air flow synchro test. It must be connected directly into the manifold and not the distributor line although this could be teed in on post 75 cars where the vac unit is connected directly to the manifold and not the carb. There is a lot about tuning with a vac gauge on the web, it's one of the most useful diagnostic tools you can own.
Iain MacKintosh

Thanks Iain. I don't know where I am looking on the manifold. I'm guessing it's obvious, but my heatshield seems to be in the way? If that's where I should be looking.

Does anyone have a picture or something of what I should be looking out for ie. t piece etc.?
M J Kirkham

Iain,

thanks for your info, do you have links to vac gauge tipps on the web?
Joern-M.

Joern, have a look at http://www.centuryperformance.com/vacuum.asp as there is quite a lot of good information there.

The inlet manifold has a couple of hex plugs fitted. REmove on of these and connect the vac gauge via a suitable connector and reducer or alternatively drill and tap the hex plug say 1/8" BSP and fit a small dia take off for the vac gauge. These are behind the heatshield which should not really be in the way.
Iain MacKintosh

Joern,
A good tip I got from this board was to take the electrical diagram from my manual, take it to Kinko's and have it blown up ~2' x 3' and laminated. I also had the legend blown up to ~ 12" x 18" and laminated. It turned out great, then you use dry erase markers to trace out any circuit your working on. I figured with the elctrical system needing attention periodically and if I ever get to a new harness, then it will be well worth the $10 investment. I have mine pinned to the drywall in the garage and I can easily read all the text and codes. Great tip from this team of experts.
Best regards,
Doug
DT Toms

Thanks again. Will try and find the correct point and then purchase a t piece so I can push the tube on.
M J Kirkham

"Guess I should study those wiring diagrams again. If only I could find some colour ones it would be easier to understand, rather than using my haynes manual. "

For good, easy to read, color wiring diagrams, in an 11x17 size, see http://www.advanceautowire.com

Click on the "stock schematics" button.

These can also be blown up at Kinko's to a much larger size if you wish.
Dan Masters

There is a pic here of my carbs:

http://asciimation.co.nz/pics/page2.html

It's a bit small sorry but you can see on the intake manifold a large connection at the back which is for the boosted brakes and just in front of that is a smaller one I am going to use for the vacuum gauge tube. Originally this was the port for the vacuum advance on the distributor but I modified my rear card to move that pickup point back to the carb near the throttle butterfly.

My manifold actually has three other ports all closed with bolts and copper washers.

Simon
Simon Jansen

Ahhhh, I see. Yes I have a few which just have bolts in them. Am I right in thinking I can just purchase a screen on peice with a push on valve end and connect the tube to this?
M J Kirkham

Also, great site there Simon. Still having a read now (after going out and checking my manifold, which I have figured out!) and it's making me feel a little bit more confident about tackling some jobs that you have!

Thanks,

Mike.
M J Kirkham

Mike, if I can do these things then anyone can. I am just learning as I go with a set of different manuals and lots and lots of questions and advice off this site!

Simon
Simon Jansen

Haha, brilliant. You have done a blinding job, keep up the good worrk buddy.
M J Kirkham

This thread was discussed between 25/06/2005 and 30/06/2005

MG MGB Technical index

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