MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - vac gauge

Hi Folks,
now i am sure most of you will think why bother with this nonsense, but its staring at me from the bench and its pretty, and i have a bit of time on my hands and a hole etc etc - anyway, any ideas how to fit a vacuum gauge (old smiths item, supposed to help you drive economically or something - just looks nice though and is a good way of filling a PO's hole so to speak!). My possibility thoughts are to find a way of manifold fitting (MGBGT 73)such that the take off is close to the middle of the manifold - say remove one of the blank plugs and drill and tap plug to accept a take off fittting - will be off centre as regards carbs but accuracy isnt going to be that good anyway is it?) Or, do I tap into the vac pipe for the distributer advance with a t-piece (then i guess is reading that carb only, but then they are balanced, and this would be easy to do), or do i have to buy a fancy carb spacer with a take off as sold by Burlen - in which case is it more sensible to fit one spacer to each carb, run both to a t-piece then to the guage?
or do i forget it and get back to slumping in front of the tv?
many thanks for any thoughts
Michael
michael

Drill and tap the plug in the rear end of the balance pipe on the intake manifold. This will give you manifold vacuum readings.
John H

I connected my vacuum gauge to a tee in the vacuum line going to the overdrive vacuum switch in my '67 GT. You probably don't have that option, but there should be a vacuum tap on the manifold near the rear carburetter. Don't connect to the distributor vacuum line.
On my MGA, I drilled and tapped a hole near the centre of the manifold and ran a small hose from there to the vacuum gauge.
The gauge is useful for driving economy and also as an engine condition indicator and test instrument.
Have fun!
Fraser
Fraser Cooper

Sounds like you are running ported vacuum (the line comes of the carb not the manifold) for the dizzy. The gauge needs manifold vacuum so your idea of opening one of the manifold blanking plugs is good. It may well be tapped already and if you have a look the parts catalogue you should find a fitting that screws neatly into it as later cars used manifold vacuum for the dizzy. If you order the rubber joining piece and some vacuum line you should be all set.
Steve Postins

Hi Michael,

Try these parts from www.holden.co.uk.

030.018 (vacuum hose)
070.350 (vacuum adapter for smiths gauge)

Chris.

Chris Currah

I think the difference in vacumn is only very marked at tickover and since the function of the gauge is fuel economy this does not matter, so tapping into the advance line would be OK. These gauges are fun , basically it will tell you all the time to shut the throttle, and if you follow it's advice you will improve your MPG, maybe quite a lot depending on how you drive. When you are driving for go, just ignore it!
Stan Best

Agreed re "economy gauge", but manifold vacuum will allow the use of the gauge for engine tuning and diagnostics too. Also need to make sure there is a restrictor in the vacuum line otherwise the gauge will leap around unhelpfully.
Steve Postins

Thanks all, this is very helpful. If I may, I would like to clarify/seek further information on a couple of points:
I like Steve Postins idea of using correct parts already available without drilling the manifold (purely as it saves me the time of removing the manifold etc) - had a look in Moss, and i see part 12H3754 (adaptor, vacuum) on the inlet manifold page - do you think this would screw directly into one of the blank plug holes on the inlet manifold - i presume this is the adaptor that later manifold-advance models use as referred to above?
Chris - thanks for the web site, have got tube on way!
John, Fraser and Stan - very useful information, as always you have shown me that there is more than one way to skin a cat, and I may yet be following your advice if the piece from Moss is a no-goer.
Fianlly, Steve Postins mentions a restrictor to be fitted on the vac line........can you tell me more Steve - what does such a device consist of? I had wondered about this, as looking at the gauge i cant imagine its a one size fits all as regards vacuum strength, so i presume a restrictor is adjusted until idle, well, reads idle. Would a simple screw thread clamp on the vac line suffice, or do i need something a little more technical.
Chaps - thios is all very much appreciated, as i realise its not really a 'serious' issue, more of a distraction. I will buy you all a virtual beer.
thanks
Michael
michael

12H3754 looks like the right part, and you'll need 12B2095 rubber unions to connect to the vacuum pipe. The restrictor dampens the signal; without it the needle waves madly and hammers against its stops. You'll soon see if you need one (you do on my Smiths gauge). A clamp may work fine but I guess might give falsely lowered signals, if you are taking the gauge scale as gospel?
Steve Postins

I'm about to fit one as well to the V8.

Have a look at:

http://www.classic-cars-online.co.uk/Archive/vacuum%20gauge.html

A nice explanation of how to use one for tuning your engine.

Liam
Liam

I am fitting one to my car too. I ended up drilling a port in my rear carb to take the vacuum advance line from there (the car is de-americanised and has HIF carbs fitted now) which leaves me with a nice little take off point on the manifold. I was going to attach the vacuum gauge to that.
Simon Jansen

Don't know about UK manifolds, but by 73 most US ones had a number of holes in the top of the balance pipe. One big one for vac booster, a couple of smaller ones tapped 5/16-24 with bolts in them for plugs - one of these was used for the manifold vac advance on cars with that, and one for emissions or PCV valve. Any hole into the manifold between the throttle plates and head will do.
Restrictor should not be necessary with standard cams, and fluctuations are a tuning point showing sticky valves for one thing; if restriction is needed, any small hole around 1/16 should work.
FRM
FR Millmore

Thanks all. To FRM - I indeed have a manifold with a number of blanked-off ports in the top, with only the big one on the end used at present (for brake servo). As i have carb vac advance, I will as you suggest use a port that is currently blank, and put in a take off designed for manifold vac advance. Will give it a go with no restriction, and if too flappy will restrict - thanks for the tip on fluctuations being a sign of sticky valves, will keep that in mind.
Cheers all of you for your advice, very kind of you for such a non-essential enquiry.
all the best
michael
michael

Don't drill the manifold, drill one of the blanking plugs, preferably one near the middle for good balance between cylinders. Whilst it's true that when driving manifold and carb vacuum give the same readings, and so it is immaterial for economic driving, one of the main features of the vacuum gauge is its diagnostic and tuning ability, for which it needs manifold vacuum. I have a Redex gauge that I bought in the 70s, this has a brass screw clamp on the larger plastic pipe just behind the gauge that allows one to adjust the sensitivity - sensitive for tuning and diagnostics, less so for driving. Economical (but not necessarily slow) driving techniques become second-nature after a while, I haven't had the gauge on a car other than for experimentation for years but still get good economy.
Paul Hunt 2

This thread was discussed between 10/10/2006 and 13/10/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.