power tune the intake system and distributor timing together for max performance! using the most important over looked tool in your shop!
I start w/ a QUICK 3 STEP SYSTEM diagnosing a customers engine 100% ACCURACY! w/ this magic forgotten tool and its sister tool!
____________Vacumm-Gauge_Testing____________
1.. Connect the Vacuum Gauge as close as possible to the intake manifold.
2.. Start the engine and let idle until reaching normal operating temperature.
3.. Adjust throttle butterfly valve and set idling speed to get a smooth engine idle.
--------- Normal ---------
This a normal reading—between 17 and 22 inches of mercury (19.5 optimum) Stock cam running at sea level. Higher elevations may cause slightly lower readings. For every 1,000 feet above sea level, you can expect the reading to be about one inch lower. You can also expect the readings to be lower for more aggressive cams.
--------- Slow, fluctuation between 14 and 22 IN/Hg ---------
Carburetor needs adjustment, air leaks, or there are other engine problems diagnosed as follows:
--------- Gauge remains at 12 IN/Hg any RPM ---------
Late valve timing.
--------- Steady Low/Extremely Low ---------
The gauge shows an extremely low reading, which holds fairly steady. This means the engine is producing less power and, therefore, less vacuum. Common causes of steady low reading are blowby due to worn piston rings or possibly late ignition or valve timing. An extremely low reading could also indicate an air leak at the intake manifold or throttle body.
--------- Steady Low/High Swing ---------
A regular swing between high and low readings often means a blown head gasket between two, side-by-side cylinders. To confirm, you’ll need to perform a compression test.
--------- Rapid Vibration at Idle ---------
If the needle vibrates rapidly between 14 and 20 inches of mercury, but then steadies as rpm increases, you may be dealing with worn valve guides. How fast the needle vibrates is telling of how many valve guides may be worn.
--------- Fluctuation Under Acceleration ---------
Conversely, if the gauge’s needle swings back and forth as the engine accelerates, your valve springs are probably too week for your engine. The swing on the gauge is usually anywhere from 10 to 22 inches of mercury, depending on the speed of the engine.
--------- Gauge pulsates @ Idle, but steady at higher RPM ---------
Indication of loose valve stem guides.
--------- Drastic Needle Drop ---------
With exhaust restriction, the vacuum gauge will often start in the normal range but will soon drop drastically as rpm is increased. You likely have a restriction somewhere within your exhaust system. When pointer drops slowly when the engine is sped up several times in secession = a clogged muffler. An unblocked muffler will cause the gauge pointer to momentarily drop to zero.
--------- Abnormally High Reading ---------
Conversely, a choked or restricted air cleaner would result in a higher than normal reading, depending on how hard the engine works to pull in air.
--------- Normal/Low Pattern ---------
When the needle drops to a low reading, returns to normal, and then repeats the pattern at regular intervals, you’re likely looking at a burnt, sticking, or leaking valve. A constant low reading on the gauge could indicate a burnt valve(s).Often, a sticking valve will cause more sporadic drops during this pattern, intermittently about 4 IN/Hg, Remove Rubber Hose, add a few drops of Marvell Mystery Oil to the manifold and test again. If the valves stop sticking, valves need to be cleaned.
--------- 4 or 5-Inch Fluctuation --------
If you notice the needle slowly fluctuating between four or five inches, chances are you have an ignition-related issue. Check your spark plug gap as this condition often indicates too narrow of a gap. Also, check your distributor cap and wires. If none of these areas are the culprit, you may need to adjust your idle mixture.
--------- 8 to 14-Inch Reading ---------
A steady low reading between eight and 14 inches of mercury is usually indicative of incorrect valve timing.
-------- gauge pulsates rapidly w/ increasing RPM ---------
it could indicate weak valve springs or poorly seated valves.
---------- Test choke Close throttle all the way and start engine. The gauge should immediately rise to 22 IN/Hg (green zone). If the gauge pointer stays around 3 to 6 IN/Hg = burnt riser tube, or the failure of the throttle valve to close