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Vacuum Gauge

Posted by barry s 
barry s Avatar
barry stoll
alexandria, VA, USA   usa
1972 MG MGB GT
1974 MG MGB
1980 MG MGB

I'd seen and heard suggestions to install a manifold vacuum gauge to get a good understanding of your engine's condition. I just installed (temporarily) a vacuum gauge. A drive around the block almost made me dizzy! The gauge moves all over the place. Step on the gas and the vacuum drops dramatically. Cruise and get high vacuum readings. What am I supposed to be looking for?
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John D. Weimer Avatar
Cape Girardeau, MO, USA   usa

It's doing right, exactly as it should. When the needle starts pulsating and jerking and jumping around while the engine is running at a steady speed is when you have some kind of trouble.
Soyokaze 72MGB Avatar
Ryan Foster
Keller, Texas, USA   usa
1972 MG MGB "Bitty"

The gauge should be steady in a certain range at idle, and go up and down as you step on the gas. Do a Google search for "reading vacuum gauge" and "MGB" to get more information on how this works. You rarely look at it when driving. It is mostly used at idle and at lights.
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twigworker Avatar
Jack Austin
Blowing Rock, NC, USA   usa

From time to time I think of installing a permanent vac gauge but the feeling soon passes.

If you had one that was fluid dampened you might be able to stand it, but a normal gauge is far too attractive to the eye to have sitting in front of you as you drive.

If you just have to use one, tap into the intake manifold at one of the nipple openings on the top of the balance tube and run a line through one of the plastic plugged holes in the firewall so that you can prop the gauge up somewhere to monitor it as you drive and put the engine through it's paces.

I use a vacuum gauge for diagnostics far less than I should. It does help to confirm my thoughts that come to mind after other diagnosis steps have been taken though. Probably the two most common aliments that can be pin-pointed with a vac gauge is an obstructed exhaust system and an improperly timed camshaft.

All an internal combustion engine is is a sophisticated air pump and different conditions affect the pumping action in different ways.

Here is simple and easy to understand chart for reference. http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

Jack



---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.



Edited 1 time(s). Last edit at 03/31/2012 08:27PM by twigworker.

Jacks Cars - Services To Interesting British Automobiles Over 40 years of caring and capable services to British cars and their owners. For help or advice please call 828-295-0224, email to jackscars@charter.net or go to www.jackscars.net.
barry s Avatar
barry stoll
alexandria, VA, USA   usa
1972 MG MGB GT
1974 MG MGB
1980 MG MGB

Jack, Thanks for the chart. I knew I'd seen something like this before. That said, I don't think I want to have this in front of my on a continuing basis.
sweep Avatar
Chris W
Gosford, NSW, Australia   aus
1966 MG MGB "Basil"
1990 Nissan Patrol "(Patrol) The Beast"
1992 Subaru Legacy "(Liberty)"

Back in the days post the oil crisis of the 70's I had a Datsun bluebird which had a big vacuum gauge next to the speedo. It was marketed as an 'Economy Gauge'. (In reality I believe it was cheaper than a tacho as the higher spec model including the TRX had a Tacho there).
Anyway I got used to having it there and telling me when I was being too much of a lead foot. I had a hole in my MGB dash that the PO had an Amp meter in and I'd already put a volt meter on the other side so I got a 'Fuel Consumption' meter and popped it in hooked as as Twig has mentioned. Didn't cost much and it's another little gizmo to look at while your checking out your temp, oil and zaps. So now you can tell how much your car sucks and blows.

Chris
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twigworker Avatar
Jack Austin
Blowing Rock, NC, USA   usa

Sucks and blows Chris? Now be nice to the old girl.

Jack



---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

Jacks Cars - Services To Interesting British Automobiles Over 40 years of caring and capable services to British cars and their owners. For help or advice please call 828-295-0224, email to jackscars@charter.net or go to www.jackscars.net.
bobmunch Avatar
Bob Muenchausen
PC, Oregon, USA   usa
1968 MG MGB GT "Traveler"

This could also help you understand what the gauge is saying as you use it. It is not just an OK dash gauge, but an even better diagnostic tool for older cars like ours.

http://www.secondchancegarage.com/public/186.cfm



Useful info about those rubber rotary seals on your crank, axles, etc >>> http://www.rlhudson.com/Shaft%20Seal%20Book/index.html

SU Carb info needed???? Go here first! >> http://www.teglerizer.com/sucarbs/index.html

A collection of photos of one of the MGB GT cutaway cars at the BMH Museum at Gaydon can be found here >> http://www.cibolas7.net/resources/cutaway.htm

A PDF copy of The British Leyland Emissions Control Manual Thru 1973 may be found here >>
https://www.dropbox.com/s/dgjgbxszv1h22hy/MG%20EMISSIONS%20MANUAL.pdf Read pages 4-1 thru 4-14 for instructions as to procedure and sequence of work.


Nuffield Australian MGB Assembly Manual >> http://mgbsmadeinaustralia.org/wp-content/uploads/2012/07/mgb_ckd_assy.pdf

And, of course, this old crock >> http://www.britishcarforum.com/bobmuenchausen/4436.html



Edited 1 time(s). Last edit at 04/01/2012 09:23AM by bobmunch.
barry s Avatar
barry stoll
alexandria, VA, USA   usa
1972 MG MGB GT
1974 MG MGB
1980 MG MGB

Thanks Bob

I had seen this some time ago but seem to have lost the link. Now I have it again. Barry
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Denny B. Avatar
Denny Brodbeck
Miami, FL, USA   usa
1971 MG MGB

In reply to # 2027792 by barry s Step on the gas and the vacuum drops dramatically.

Back in the 50's (when I was a kid) the windshield wipers on many cars were "vacuum operated". If you had your wipers on and gave it gas the wipers would STOP momentarily until the vacuum built back up.

A real safety feature! eye popping smiley
mowog1 Avatar
Rick Ingram
Saint Joseph, Illinois, USA   usa
1968 MG MGC
1969 MG MGC
1972 MG MGB "Mallard"
1974 MG MGB GT V8 Conversion
1978 MG MGB

Moss - Using a vacuum gauge to diagnose.







1968 MGC - 1969 MGC - 1972 MGB - 1974&1/2 MGB/GT V8 conversion - 1978 MGB

mowog1@aol.com

Pieces Of Eight: Struts For Your MG Pieces of Eight! has provided gas-charged strut kits for the MGB bonnet, MGB and MGC bootlid, and MGB/GT or MGC/GT hatch since 1996. Contact Rick at: mowog1@aol.com
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