Original submission by Jacks June 24th 2006:
The Vacuum Gauge:
The Vacuum Gauge can provide valuable information for optimising the running of your engine
as well as its tuning.
The only difference between a Vacuum Gauge and an Economy Gauge is the display. They are both connected to the manifold in the same location and both move to the same locations
in response to the vacuum present in the Inlet Manifold.
The larger the Vacuum Gauge's Display is, the greater its order of accuracy.
thumb|Vacuum/Pressure Gauge. Click to enlargen. Photo by HRAmbo. The Vacuum Gauge is connected directly to the Inlet Manifold,
PCV port or at some point below the Throttle Butterfly and not to the ported side (above the throttle) of the Carburettor.
Tuning the Carburettor by Vacuum Gauge:The Vacuum Gauge can be used to tune the mixture of your Carburettor as well as achieve
optimum Ignition Timing.
Important: The Static Ignition Timing must be set before the Carburettor adjustments can be done.
Don't forget to reset the Idle Speed after each adjustment.
Remember, the richer the mixture, the higher the Vacuum Reading will be.
So now with your Gauge connected correctly, place it in a position so you can clearly see it,
now if your Carburettor has been jetted correctly,
you can then now lean off one of your mixture screws on your Carburettor,
notice the Gauge will start to shudder and at the same time the Engine will shudder as well,
your engine should stop,
if it doesn't,
then the mixture is too rich,
you may need to do some other requirement to fix this,
your Carburettor may need rejetting, or more Idle Air by re-drilling the primary Butterfly,
on some Carburettors they do have an Idle Air Bleed that can fix this.
Now if you adjust the screw to richen the mixture,
you will notice the Vacuum Gauge will steadily climb,
so when it gets to the highest reading,
you then can stop adjusting the Mixture Screw,
if you have more than one mixture screw,
then you will need to repeat the procedure,
this of course will take a bit of time to grasp,
but you need to get the optimum result.
On some Carburettors that have the four corner mixture screws,
they will take even more time to set up,
so if you increase the idle to around 2500 rpm to double check your secondary mixture screws settings,
you may even have to repeat it a few times, nevertheless practice makes perfect and in the end you will reap the reward for you efforts, it just take your time, and beside you will get to know you carby workings etc.
When setting the mixture with a Vacuum Gauge it should also be remembered, that in most cases, depending on the engine the setting may in fact be a little too rich, so turn the mixture screw to lean off a fraction, say 1/16 to a 1/4 of a turn .
Another point to remember is the more timing increase the more the vacuum will be, nevertheless, by playing around with the vacuum gauge will help you to understand a bit more on the workings of your engine.
You can also learn how to drive more economically by using a Vacuum Gauge, there are some on the market that look like the one in the image. I use this one in my CF Bedford.
If you have an ordinary Vacuum Gauge, with no fancy colour section on it then that will work just as well.
The Vacuum Gauge can also provide you with some valuable information, in addition to tuning your Carburettor and Ignition.
Reading the Vacuum Gauge:
A normal (stock) healthy engine should read about 57 to 71 kpa of vacuum and the needle should be fairly steady.
This will also depend if you're at sea level or higher up,
anyway normal atmospheric pressure is measured at sea level,
so therefore the level of vacuum will be depending on your location,
so for the technical ones out there that need to be spot on here is some information to burn into the brain,
for every 300 meters increase in elevation above sea level, the gauge will decrease about three kpa.
Incidental the vacuum comes from the underside of the butterfly or throttle, so when the throttle is closed, (the idle stage), the engine is trying it hardest to pump in as much air as it can, but the engine is being somewhat restricted by the butterfly/throttle to just a trickle, so now we have a vacuum being created under the throttle/butterfly
The better the engine the more air is pumped in, the greater the vacuum reading will be, if the rings are worn etc then less air is drawn in so hence a lower vacuum reading.
so by using a vacuum gauge that is plugged into the intake system after the butterfly/throttle ,we can now get some valuable information as a guide to how good the engine is mechanically, and the way the needle moves on the gauge will indicate some additional information while the engine is rapidly running opened and closed as you moving the throttle.
Vacuum Gauge Indications:
A steady reading usual indicates a leaking gasket between the intake manifold and the Carburettor or the Throttle Body,
maybe a leaky Vacuum Hose, or late Ignition Timing, or even incorrect camshaft timing, use a Timing Light to confirm,
LINK to timing
If the reading is 76 to 203mm below normal and it fluctuates at that low reading,
then it's properly the intake manifold gasket leaking at the intake port, or if injected a faulty injector.
If the needle moves slowly through the a wide range, then check the PCV system, or maybe incorrect idle fuel mixture,
or the carburettor throttle body, or intake manifold gasket leaks.
Now if the needle has regular drops of around 50 to 100 mm at a steady rate,
then I'd suspect that the valves are probably leaking, so do a compression or leak down test to confirm,
Irregular drop or down-flick of the needle can be caused by a sticking valve, or ignition misfire, check spark plugs,
do a compression or leak down test. LINK
A rapid vibration, of around 7 kpa vibrations at idle speed, combined with exhaust smoke will indicate worn valve guides, do a leak down test. LINK
however if rapid vibration occurs when the engine speed is increased, then check for head gasket or a leaking intake manifold
gasket, weak valve springs, burnt valves, or ignition misfire.
A light fluctuation of around 25 mm, up and down, may mean ignition problems, if there is large fluctuations,
then suspect dead cylinder or a blown head casket, do a compression or leak down test to confirm, LINK
Also Check for a slow return, after you have quickly snapped the throttle open to say around 2500 rpm,
then let it shut, the needle should drop to zero, then rise above normal idle reading over 10 KPa
and then return to the previous idle reading, now if it returns slowly,
and it doesn't peak when the throttle is snapped shut, then its bad news as the rings may be worn,
however if there is a long delay it could be a restricted exhaust system, muffler or catalytic converter,
you could then momentarily disconnect and do the test again to confirm if need be.
The above is a way of using your Vacuum Gauge, as we want to get the Highest reading we can from the vacuum gauge as possible when Idling, so it will pay to have one on your dash , as I believe it an essential part of your dash display,
a very useful tool indeed, apart from giving information on how your engine is running,
It can be used to help you get maximum fuel economy as well so will pay for itself in a very short time.