From Here

Using a Vacuum Gauge

Tuning with a Vacuum Gage

Many racers and engine tuners have opted for state-of-the-art diagnostic equipment but have forgotten one of the simplest, as well as one of the most accurate tuning tools...... the vacuum gauge.

When using any vacuum gauge keep in mind that they are all calibrated at sea level and read-outs in the instructions are in reference to that level. When above sea level all readings will drop one division per each 1,000 feet of altitude above sea level. Thus a reading of 20 inches of vacuum at sea level would drop to 19 inches at 1,000 feet, 18 inches at 2,000 feet, etc. All readings are taken at idle except as noted.

If your engine is barely able to sustain 2"Hg vacuum at idle, it will be hard to tune using this method, but it is still useful. In most cases if you use a quality large faced Vacuum gauge, with some experience you can effectively tune your car's fuel and timing systems. Tuning with the engine running compensates for wear in the timing gear and valve train, and therefore providing better results than with the manufacturers recommended settings.

Note: All adjustments are made with the transmission in PARK or NEUTRAL, with the exception of Idle Speed, which is set in DRIVE.

Caution: Make all adjustments from the side of the vehicle, rather than standing in front of it.

Vacuum Gage Connection

Connect the gauge to a "manifold" vacuum source, NOT "ported" vacuum, that rises as RPM increases. In most cases this will be a direct manifold source or possibly the PCV port (larger port) on the carb.

Start by first warming the engine and note the idle vacuum reading. Normal vacuum at idle should be 19-21 inches for a six cylinder, or 15-18 inches on a low compression engine. Pre-set the ignition timing so that it is close to the manufactures recommended setting, before making any carb adjustments.

Carb Adjustments

To adjust the carb, start by leaning out one of the mixture screws (turn in) until the gauge as well as the engine begins to shudder. Next bring the screw back towards rich (turn out), while watching the gauge. Stop adjusting when the gauge reaches it's highest reading. Then do the same process for the other mixture screw. After each adjustment is made, reset your idle speed.

Small adjustments are best, and in fact "optimum" carb settings on the vacuum gauge (highest reading) is usually richer than it needs to be. In other words, once the highest reading is reached, back-off (or lean) the adjustment approximately 1/4 turn in. You may have to repeat the process a few times to get optimum results, but it's worth the time and effort.

Note: With a properly jetted carburetor, turning either of the mixture screws all the way lean, should kill the engine. If not, you're too rich! This may require re-jetting, or drilling the primary butterflies to add more idle air. Many of the newer "race" carbs allow you to change Idle air bleeds to fix this.

For carbs with 4-corner mixture screws, you have to take a bit more time. You can also run the engine at a "steady-state" RPM of say, 2500 RPM to double-check your secondary mixture screw settings. Do this with temperament! It takes time to get used to what you are seeing as well as if it is actually helping. Each engine will behave differently.

Remember to rev the engine to clear the spark plugs before taking your final reading and readjust if necessary. When you blip the throttle, the needle should drop to as low as 2, pop back up to as high as 26, and quickly level off in the normal zone.

Ignition Timing

After the carb mixture is set, you can proceed with the ignition timing. Slacken the distributor clamp bolt, and with the engine still at idle, advance or retard the ignition until the highest steady vacuum reading is obtained. Then retard the timing until the vacuum gauge reading drops slightly, approximately one half to one inch. In some cases, you may need to retard the timing up to two inches to prevent pre-igniton (pinging).

Note: If you can't get the reading into the "normal" zone by adjusting the distributor then valve timing is the problem. Timing with a vacuum gauge will normally result in timing that is more advanced than what specifications call for, so you need to be alert for any pre-ignition (pinging) in the engine and adjust accordingly (retard the timing if needed).

Your engine should now be tuned to perfection. Vacuum Gauge Needle Readings - Explained Steady needle: Normal reading (usually 17-22" Hg. in stock engines). Race engines vary "a lot" and in most cases will be considerably less. Intermittent fluctuation at idle: Ignition miss, sticking valves, hyd lifter bleeding off, or just a BIG camshaft. Low, but steady reading: Late timing, incorrect valve timing, low compression, sticking throttle valve, carb or manifold vacuum leak. Drifting needle: Improper carb setting or minor vacuum leak (adjust carb). Fluctuating needle, as RPM increases: Ignition miss, blown head gasket, leaking valve or weak or broken valve spring. Steady, but needle drops regularly: Burnt or leaking valve, or incorrect valve setting. (needle will fall when bad valve operates). Gradual drop at idle: Clogged exhaust, excessive back-pressureing with Vacuum (in extreme cases the engine will die at idle). Excessive vibration that steadies as RPM increases: Worn valve guides.

Sticky Valves
A 3 or 4 point intermittent drop of the needle indicates a sticking valve(s). A quick double check of this condition is to shoot some penetrating oil through the carburetor while the engine is running. If the needle steadies up in the normal zone a while and goes back into the intermittent drop you have a/some valve sticking problem(s). This isn't a big deal and you can cure it quite easily. Just start adding some Marvel Mystery oil or Sea Foam to the gas as directed on the and the situation will be remedied in short order. Tight Tappets or Burnt Valves These conditions are indicated by an intermittent quick dropping of the needle into, as low as, the 14 inch zone. The drop and rise will be quick and consistent at idle. If you see this set the tappets and run the test again. If you get the same, or nearly so, readings again you have a burnt valve.

Note: You could get the same above readings caused by carbon build-up on the valves. Try running a can of Sea Foam through the intake, as instructed on the can, before committing to pulling the head for no good reason.

Weak Valve Springs
Weak springs will show as a normal vacuum reading at idle but a wildly jerking and fluctuating needle when the engine is revved up and running steady. The range of fluctuation can be as much as form 10 inches to 22 inches.

Worn Valve Guides
If the needle fluctuates with jerky motions somewhere in the 14 to 21 inch range at idle but steadies as speed in increased the valve guides are worn.

Leaky Head Gasket
A jerky fluctuating needle in the 8 to 20 inch range is a good indicator of a leaky head gasket, but could be a combination of the above listed valve problems ganging up on you. In any case you'll end up pulling the head and pinning it down before long.

Choked Muffler
A normal reading at idle with a gradual drop to 0 or very near it as the engine is speeded up is caused by a choked muffler, clogged cat converter, or any stoppage of exhaust flow.