@alsancle
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Atmospheric pressure is measured at 14.7 pounds/square inch at sea level. That pressure corresponds to the weight of air holding a column of mercury 29.92 inches in height. Vacuum is by definition: pressure below normal atmospheric, commonly caused by a suction that is taking the air molecules away from a particular location. In engines, of course, the air is being sucked in by the vacuum created by the movement of the pistons.
All vacuum gauge faces are graduated in inches of mercury, although some have additional scales in millimeters of mercury. For those interested in international equivalents, one millimeter of mercury = one Torr. Further, one atmosphere = 760 torr = 1.013 x 105 Pa (Pascal).
Note also that most vacuum gauges are equipped with an additional scale that measures fuel pump pressure. This allows the user to connect the hose directly to the fuel line entering the carburetor to measure the fuel pump's pressure. Since many fuel system problems can be traced to the pumps themselves, such a tool can save a lot of troubleshooting time. Is this a great tool, or what?
The most important thing to do when using a vacuum gauge is to connect it to a constant vacuum source on the engine. Some manifolds incorporate a plug that may be removed for such purposes. If none exists, the next best place to connect is the PCV hose. If that's too hard to reach, connect to the power brake vacuum hose (on the engine side of the one-way valve in the hose).
Lastly, you can connect to the vacuum line at the carburetor, but make sure the line has vacuum at idle. Many distributors were designed to get advance vacuum only when the throttle plate was opened, in which case there was no vacuum at idle. Make sure you've connected things properly.
Note: The readings following will be typical at sea level. In general, subtract one inch for each 1000 feet above sea level. Also, these readings are typical examples seen in stock engines (except where noted,) not absolute "drop-dead" numbers for your engine! The idea here is to use the vacuum gauge to see how far your engine is differing from normally expected readings. Your gauge's needle might move at slightly different rates, so don't be concerned. You need only recognize a "flicker" from a "sweep."
The readings in Scenario 1, 2 and 3 below are normal readings taken from a properly functioning engine. The remaining Scenarios represent engines with various problems, as explained in each Scenario.
Scenario 1. Normal Vacuum with a steady reading of 15 - 22 inches.
Scenario 2 Rapid acceleration the vacuum will initially drop to close to zero.
Scenario 2 Continued...
If the Engine is then decelerated (from the accelerated position) the vacuum will momentarily go up.
Scenario 3: High performance engines will show a vacuum reading lower than stock, typically around 15 inches.
Scenario 4. Engine with worn rings or diluted oil.
Normal operating shows 15 to 17 inches. Rapid acceleration causes needle to drop to zero, Rapid deceleration causes need to only rise to 20-23 instead of the normal 26 to 30.
Scenario 5 Sticky Valves.
If the problem is sticky valves, the needle remains steady, but quickly flicks down and then back up. The drop downward is usually around 3 to 4 inches on the scale.
Scenario 6. Burned or constantly leaking valves.
There is an evenly spaced downward flicker of the needle.
Scenario 7. Poorly seated valves.
A regular down flick over 2 to 4 inches indicates poorly seated valves.
Scenario 8. Worn Valve Guides.
When you see the needle regularly swing back and forth between 4 to 6 inches it means the valve guides are worn.
Scenario 9 Weak Valve Springs.
If the needle oscillates violently over 10 to 14 inches as the engine speed increases that means weak valve springs. At idle the needle may be steady but as you slowly increase throttle it beings to oscillate.
Scenario 10 Late Valve Timing.
A steady reading of 10 pounds indicates late valve timing or a possible slight intake leak.
Scenario 11 Retarded Ignition Timing.
A steady but mediocre reading of around 15 inches indicates retarded ignition timing.
Scenario 12 Intake Leak
A low steady reading of 3 to 6 inches indicates an intake leak.
Scenario 13 Blown head gasket.
If the needle starts off at normal vacuum of around 20 inches and then drops off in a regular manner suspect a blown head gasket.
Scenario 14 Clogged Exhaust.
If the needle starts off in the normal spot but slowly goes to zero as the engine speed increases suspect a clogged exhaust.
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