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Vacuum Gauge

Posted by barry s 
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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?

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-----------------------------------
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-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

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

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.

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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

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.

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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



"All that my contradictory experiences demonstrate, sometimes, is that things and materials don't always follow the course of our commonly accepted thinking about them, but end up sometimes besting our knowledge of them."


SU Carb setup instructions ~ You don't need smog gear to need/use these factory instructions.
>>>https://www.dropbox.com/s/dgjgbxszv1h22hy/MG%20EMISSIONS%20MANUAL.pdf

MGB GT cutaway car photos can be found here >>> http://www.cibolas7.net/resources/cutaway.htm

A thorough guide to rubber rotary seals as used on your crank, axles, etc >>> http://www.rlhudson.com/Shaft%20Seal%20Book/index.html



Edited 1 time(s). Last edit at 2012-04-01 09:23 AM 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

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 "The Green C"
1969 MG MGC "Vicky"
1972 MG MGB "Mallard"
1974 MG MGB GT V8 Conversion "The V8"
1978 MG MGB "Maggie"

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


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