Using a Vacuum Guage 

Description Hook it up, Analyize the Readings
Author elcamino121 Date Tue Jan 23, 2007 6:16 pm Type Tech Article
Category Documents
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Vacuum Gauge Reactions

Connect the vacuum gauge hose to the intake manifold. On many engines there is a plug in the intake manifold, which can be removed. If this is the case, remove the plug, screw the proper adapter fitting in its place and re-connect the hose(s). On engines that do not have plugs, remove a hose from a vacuum point between the carburetor and the head, and connect the vacuum gauge in its place, using a β€œT” fitting. On multi cylinder engines, connect one gauge to each cylinder. Due to the individual cylinder characteristics, it may be necessary to install an aquarium air control valve in each line to diminish the natural pulses occurring in the various strokes of the engine, just be sure you don’t fully restrict the flow.

1.When the pointer is low but steady, the trouble is something that affects all cylinders alike. This could be late ignition timing, late valve timing, or loose tappets. Such troubles can be responsible for the reading being anywhere from Ό to 2” low. Intake system leaks (warped intake manifold, leaky manifold gasket, leaky carburetor flange gasket, poorly seating intake valves and leaky intake guides) can be responsible for the reading being anywhere from Όβ€ to 16 β€œ low depending on the size of the leak.
2.When the pointer pulses regularly, the trouble is something which affects one particular cylinder consistently; for instance, one exhaust valve which does not seat, one exhaust tappet which is too tight, or one sticky valve, or one dead spark plug. The pulses of the pointer are much greater if they are caused by some form of leakage such as exhaust valve not seating, rather than something that only slows the pistons, such as a dead spark plug.
3.An unsteady pointer may be caused by any of the defects mentioned in # 2, but with these defects occurring irregularly and in several or all of the cylinders. Other defects, which are responsible for an unsteady pointer, are; a rich carburetor mixture, extremely advanced or retarded spark, wide spark plug gaps, intake valves shifting on their seats.
4.If the pointer is unsteady and if the sweeps of the pointer increase with increases in speed, the trouble is weak or broken valve springs. If the sweeps get smaller but more rapid on increasing speed, the trouble is in the intake system leaks. If the pointer steadies on increasing speed, the trouble is ignition, or carburetion.
5. A near full swing will occur when the throttle valve is opened and closed (blipped). If the pointer does not respond with wide sweeps it is an indication that leakage exists. If the possibilities of leakage through poorly seating valves, manifolds, gaskets, etc. are exhausted, then the piston rings are not sealing properly.
6.On older ignition systems (points and condenser), when the ignition is advanced to the highest steady reading and then retarded so the pointer is Όβ€ to ½ β€œ below the highest steady reading, and if the pointer is within specifications (+/- 20”), and holding steady, and if the ignition is in time (check with timing light), then the engine can be considered to be β€œin time”. Allow 1” less reading for each 1000 feet in elevation.

Normal Reactions

18” to 22 β€œ is usually referred to as the normal range for a warmed up engine at idle speed. However, with greater and greater degrees of valve overlap, this range is becoming meaningless. Some manufacturers are now accepting 15” as an acceptable reading at idle. The goal that you want to achieve is the highest equal reading on all cylinders at the correct idle speed.

Responses of the gauge to movement of the throttle are much more exaggerated in a driving test than in a floor test but in either case, the vacuum will fall off when the throttle is suddenly opened and it will rise when the throttle is suddenly closed. In a floor test, it is normal for the pointer to oscillate between 3” and 25” as the throttle is suddenly opened and closed. If the vacuum gauge does not respond in this way, it is an indication that leakage exists either in the form of poorly seating valves or in the form of poorly sealing piston rings.

Reactions to Speed Changes

Adjustment of ignition timing was discussed in a previous paragraph, so it should be understood as one method by which the vacuum reading can be changed without disturbing the throttle valve position.
Just as a vacuum gauge may be used to adjust ignition timing, it may be used, in much the same manner, to adjust the idle mixture screw in a carburetor by richening the mixture to the highest steady reading. Thus, a faulty carburetor adjustment will affect the vacuum reading also.
Valve timing too, affects vacuum gauge readings. Valve timing can be late because of improper installation of timing gears, but more common than this, tappets are set too loose, causing the valves to open late (and close too early). This condition would reduce power considerably. Consequently, the speed of the pistons would be slowed and this would be still another reason why the vacuum gauge reading would be low.
Speed of the pistons will be slowed by anything that reduces power, such as shorted (fouled) spark plugs, leaky ignition wires, or any number of ignition difficulties, which may ground out a cylinder. To learn the effect of a dead cylinder, connect the vacuum gauge to an engine and short out first one then the other (2 cylinders) or 1,2, 3 cylinders on a 4 cylinder machine and notice the effects.

Reaction to Leakage

It should be understandable that a leak in any of the passageways between the carburetor and the cylinders will interfere with the flow of air and fuel from the carburetors to the cylinders. Defects of this nature affect the speed of the pistons too; but much more directly they interfere with the actual creation of vacuum. A leak in the manifold gasket (depending on it’s size) can be responsible for a reading anywhere from slightly above zero to slightly below normal. Leakage into the manifold has much more effect on the gauge reading than defects that affect piston speed only. A very tiny leak will lower the vacuum reading considerably.
Intake system leakage is not only found in the form of a leaky manifold gasket. It might be in the form of a warped or loose intake manifold, leaky carburetor flange gasket, worn throttle shaft, one or more poorly seating intake valves (for any reason including a tight tappet), one or more worn intake valve guides, or leaks in any of the lines or fittings to any vacuum operated units such as fuel tank diaphragms or fuel mixture control valves.

Isolating the Trouble

Referring to paragraphs #1, #2, and #3, note that all abnormal reactions of the gauge can be put into one of three groups. In paragraph #1, the reading is steady but too low. In paragraph #2, the reading is fairly high, but the pointer pulses. In paragraph #3, the reading may be high or low but the pointer is unsteady. If the vacuum gauge reading style falls in paragraph #1, and especially if it is very low, concentrate on finding intake system leaks. If the pulses are large, it is most likely intake system leakage affecting that cylinder. If the pulses are very small it may be ignition. An unsteady reading is perhaps the most common reaction and probably the most difficult to analyze. Since paragraph #2 is a form of paragraph #3, isolation of the trouble nearly always boils down to doing one of two things or both; (a) finding out why the pointer is unsteady, (b) finding out why the reading is low.

(a)Unsteady Pointer

If the Pointer moves in wide sweeps of 3”-4”, the trouble could be a blown head gasket, or weak valve springs. An unsteady action of the pointer may also be caused by unevenly or widely spaced spark plug gaps or spark plugs that are carbonized (fouled) from a rich mixture or from oil; or just because they are too cold for the engine. An unsteady pointer may also be the result of small inconsistent intake system leaks, such as intake valves shifting on their seats because of worn guides or such as a worn throttle shaft in a carburetor. Insufficient tappet clearance, weak valve springs and sticky valves also cause the pointer of the gauge to be unsteady.
Often the trouble may be isolated by adjustment of the timing and the carburetor and a check of the ignition circuit including the plugs. If further isolation is necessary, the engine speed should be increased to what would be about 12-15 MPH. The higher speed reduces the effect of a rich mixture and decreases the effect of small intake system leaks, at the same time; it increases the effect of bad valve action.

On the faster engine speed, if the pointer steadies, the trouble is ignition, carburetion, or advanced timing. If the sweeps get larger with further increases in speed, the trouble is definitely weak or broken valve springs.

(b)Low Reading

A low reading can often be corrected too, by adjusting the ignition timing. If it cannot be corrected in this manner, concentrate on finding an intake system leak. Running the engine and listening for the escape of the burned gases can locate head gasket leaks to the outside atmosphere. If the escape is suspected to be through the spark plug, the plug should be examined closely for streaks on the ceramic part. If the head gasket is blown, or if the exhaust valves do not seal properly, a compression gauge test will reveal the problem.

To determine if intake system leaks are present, the ignition should be shut off and the engine cranked with the starter while a hand is held tightly over the carburetor (after removing the air cleaner). A vacuum gauge reading during cranking of at least 17” indicates the intake system to be fairly well sealed, but the higher the reading the better. If the reading is low it should be determined whether the leak is in the carburetor or in the engine. To do this, the carburetor should be removed and the test made again but this time with the mouth of the manifold covered with a hand or some kind of a stopper. If a higher reading is obtained by the latter test, then the leak is in the carburetor. Possibly a worn throttle shaft, a poor vacuum line, a leaky flange gasket or a cracked casting would be the source of the trouble. With the engine running, intake system leaks may be detected by squirting oil, WD-40, or propane around where leaks are suspected. If a leak is present, the oil or WD-40 disappears; the engine speeds up with the propane.

User comments 
mark75 Posted: Tue Jan 23, 2007 7:44 pm    Post subject

Great info.
I had a friend in high school that was such a big fan of vacuum guage he mounted one on the dash of his '50 Merc. That old flat-head always ran like a champ, now maybe I know why.
grazingazer Posted: Tue Jan 23, 2007 10:51 pm    Post subject Re: [ KB ] Using a Vacuum Guage

vacuum,oil pressure and ammeter are the most important gauges you can have on a mo'cycle in my opinion
they'll save ya alot of hairpullin hairpuller BurnOut
Matthew Posted: Tue Jan 23, 2007 11:08 pm    Post subject

Ever seen the "mileage" gauges you could buy.They were just a vacuum gauge with a easy to read for dummies chart.

Use to have two but now I only got 1 but I have it hooked up to what ever car I'm driving.

Them fancy mileage things in car that say how many MPG your getting right then, work your way back and you find the computer is just using vacuum.
grazingazer Posted: Wed Jan 24, 2007 9:02 am    Post subject

Matthew wrote (View Post): › Ever seen the "mileage" gauges you could buy.They were just a vacuum gauge with a easy to read for dummies chart.

Use to have two but now I only got 1 but I have it hooked up to what ever car I'm driving.

Them fancy mileage things in car that say how many MPG your getting right then, work your way back and you find the computer is just using vacuum.
if you use two you can synchronize your carbs
Matthew Posted: Wed Jan 24, 2007 2:06 pm    Post subject

Why I use to have two, then one went MIA

Got them real cheap like $7 each even with pressed in inserts to cancel out vacuum pulse at

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