The ignition system is the part of your vehicle/engine that ignites the compressed fuel and air mixture in the combustion chamber. The system consists of a voltage supply (12-volt battery), an on/off switch (ignition switch), a distributor, a triggering device, ignition coil, spark plugs, and spark plug wires.

The 12-Volt battery supplies the primary voltage to the coil and ignition unit (if electronic). When the coil is activated by the trigging device (contact points, electronic module, crank trigger, magnetic reluctor, or other type of trigger), the coil builds and releases a secondary voltage which is sent to the distributor via the coil wire. It does this at a very high rate of recovery speed. The secondary voltage (or spark) travels through the distributor to the appropriate spark plug by means the rotor and a plug wire, where it jumps the spark plug gap and ignites the mixture.

Ignition Timing is the relationship (or the position) of the piston and when ignition spark occurs. If the ignition spark occurs too soon, it can actually push against a piston while it is compressing the fuel/air mixture, resulting in detonation, power loss, higher combustion temperatures, backfires, and early engine failure. If the spark occurs too late, the ignition occurs when the piston is traveling back down the cylinder, resulting in wasted energy, unburned fuel, higher emissions, and power loss. Therefore it is obvious that the ignition spark must occur at precisely the right moment to achieve maximum power and efficiency.

On older engines like our inline sixes, and most race cars with aftermarket distributors, the ignition system uses a simple mechanical advance (or a combination of mechanical and vacuum advance) built into the distributor. As RPM increases, the mechanical and/or vacuum advance (weights and springs) moves due to the spinning inertia occurring within the distributor, thereby advancing the ignition timing in relation to piston location based on engine speed. Both the initial and the advanced timing, or a combination of (total timing) can be adjusting for your engine's optimum performance, which is referred to as the timing curve. Initial (or idle) timing is set with a timing light by looking at the balancer mark and timing tab indicator, and rotating the distributor. Advanced timing is adjusted by changing the weights and/or springs inside the distributor.

If the initial timing is set at 12°, the spark is reaching the spark plug when the piston is
12° before top dead center (BTDC) at engine idle. However as engine rpm increases the internal engine parts are moving faster, therefore you must get the spark in the cylinder sooner in order to ignite and allow the fuel air mixture to burn completely for optimum power and efficiency. But not too soon so that it causes detonation or pre-ignition.

Say for example that you have that 12° initial timing and 24° of advanced timing, you would have a "Total" timing of 36°. However the 24° of advanced timing does not happen all at once, as it occurs gradually as engine rpm's increase. Remember, as engine rpm increases, so does the inertia against the distributor weights within the distributor. The springs holding the weights, is the opposite resistance force that controls the speed, or rate, that the weights move outward which advances the timing. By changing the size of the weights, you change the amount of advance that is applied. By changing the spring pressures, you change the rate at which the advance is applied. A lighter spring will make the advance occur faster, while a heavier spring will slow the advance rate.

Ported Vacuum: Older OEM applications used "ported" vacuum. Ported vacuum means the port is drawing vacuum "above" the throttle blades in the carburetor. This means that as RPM increases, vacuum increases and in turn, vacuum advance increases.

Manifold Vacuum: When vacuum is drawn from a point below the throttle blades, or from the intake manifold, this is called Manifold Vacuum. A good running street engine will have a measured manifold vacuum (at idle) between 14-20 inches. Manifold vacuum will give you a ton of advance at idle, but as load increases, vacuum drops and timing is reduced.

Aftermarket vacuum-advance units typically employ an Allen screw inside to control advance rate. Turn the screw clockwise to slow the advance rate; while turning it counterclockwise quickens advance rate. Ford's original vacuum-advance units had shims inside and a spring designed to control the advance rate. Add shims to slow the advance rate, or use fewer shims to quicken the advance rate.

Up-graded ignition systems allow for better efficiency, faster starting, higher RPM, lower emissions, and result in smoother and overall enhanced performance gains.

Coil: The ignition coil is the workhorse of the ignition system and must be the first upgrade you consider. OEM or coils not designed for your application can overheat, cause misfires, boil over (possibility of causing a vehicle fire) and otherwise just cause you grief. You must have a coil designed for electronic ignition if that is what you are using. Also, there are a few coils on the market that are for "Limited Use Racing" as in Drag Racing or short term use. These coils are designed to give Maximum Output, but cannot safely do it for extended periods. You should not use coils with this designation on the street or in circle track applications.

Distributor: Up-grade to an Electronic or Magnetic Trigger distributors and get rid of those prehistoric point style distributors. A performance distributor will give you better control of timing curves and triggering accuracy.

Wires: If you have anything that is electronic on your vehicle (computer, ignition unit, triggering module, radios, etc) you MUST use spiral core wire sets. There's not much gain going to solid core wires unless you are using a Magneto. With the low resistance of the spiral core wires it is ridiculous to take a chance using a solid core wire and damaging another component on your vehicle.

Ignition timing needs to be checked and tuned at least two different ways. First, static ignition timing should be checked with the vacuum-advance hose disconnected. Follow factory specifications for initial timing.

With the vacuum advance reconnected, check total timing, which can be done two ways. Total timing is checked at 3,500 rpm with the vacuum advance connected. Ideally, push for the maximum number of degrees before BTDC without detonation (pinging or spark knock). In most circumstances, total advance should be 34-36 degrees BTDC.

The rate of advance is checked by goosing the throttle to wide open and watching the timing mark with a timing light. If spark advance comes on too quickly, the engine will tend to break up (misfire) and ping (spark knock), which means you need to slow the rate of advance. If the engine noses over and doesn't rev quickly, speed up the rate of advance.

When the vacuum advance is dialed in just right, the engine revs smoothly and crisply. On the road, power comes on quickly without hesitation or spark knock. At sustained highway speeds, the engine should maintain a smooth demeanor without misfire. If you feel a slight misfire at cruising speeds or hear pinging under hard acceleration, you have too much timing.

Another area to check is manifold vacuum. Healthy manifold vacuum is normally 15-22 inches at idle. Checking manifold vacuum is just as important as checking compression and cylinder leak-down. It determines an engine's state of health. Note that manifold vacuum gets tricky when the engine is fitted with an aggressive camshaft or too much carburetor.

Hook up a vacuum gauge to your motor so you are reading manifold vacuum ( full vacuum at idle, etc.) and remove the vacuum hose to the advance diaphragm. Set your idle speed just a little lower than normal by about 100 rpm, if the motor will stay running. Advance the timing while watching the vacuum gauge, when you get to a point of the highest vacuum reading, stop advancing the timing and start to retard the timing until you see a reduction of 2 inches of vacuum, from the highest reading. Lock down the distributor and hook up your timing light and see how close you are to your preferred initial timing setting. This is how mechanics did it in the 50's for a quick setting.... some still prefer to do it this way.

For more information, see our tech article "Tuning with a Vacuum Gage".

Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.

Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.


So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.

Why an engine needs more advance as its speed increases.
When the compressed mixture inside a cylinder is ignited it takes time for the flame front to reach the piston and for the expanding gases to start pushing it down. The time that this takes changes according to a number of variables such as mixture strength, how well the cylinder has filled (dependent on volumetric efficiency and throttle opening), compression ratio and combustion chamber shape. Given the same cir***stances of mixture strength, cylinder filling and CR, the time taken for the mixture to fully ignite and burn is the same regardless of engine speed. At increasingly higher RPM however, the time available for this burn to take place is correspondingly less, so it follows that you have to start burning the mixture earlier in order for it to push on the piston at the right time. This is the basis for increasing ignition advance.
Too much of this and the burning mixture hits the piston as it rises (pinking or pinging), too little and the flame front reaches the piston far too late and does not do a good job of pushing the piston down and the engine behaves like a herd of turtles. One of the reasons a diesel engine does not perform at higher RPM is that it has compression only ignition, so there is no way to increase the effective ignition advance.

How this is achieved.
The distributor as fitted to conventional ignition systems does not just distribute the spark amongst the cylinders and switch the coil; it also contains a centrifugal mechanism that advances the ignition timing automatically as engine RPM rises. Normally there are a pair of weights within the distributor which under the affects of centrifugal force tend to be thrown outwards, this tendency is greater as RPM increases. The weights are shackled by two small springs that restrain them progressively. As the weights move outwards they exert a turning force on the top of the distributor shaft relative to the driven part of the shaft, this moves in the same direction as the distributors rotation thereby causing the points/electronic trigger to actuate earlier and advancing the ignition timing. As engine speed increases the weights overcome more of the spring's tension and advance the timing still more. There is normally a stop of some kind that limits the amount of advance that the distributor can supply. This centrifugal mechanism is usually hidden away underneath the base plate of the distributor.

In this article we're going to focus primarily on carbureted, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.

When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.

Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.

So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Petronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.

Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.

When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.

Initial
This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.

Mechanical/Centrifugal
Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpm's, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.

Total Advance
So far we have looked at initial advance and mechanical advance. Both of these combined gives total advance. Say for example initial was found to be 6 degrees, and we visually verified that the reluctor arm was on the 15L side. Total timing, theoretically, is then the initial + mechanical. In this case 6 + (15 x 2) = 36 degrees. If we shined a timing light on the marks (with vacuum hose disconnected and plugged), at idle we'd see 6 degrees, then as we increased the engine speed, we'd see more and more advance, until at some point the total centrifugal advance would be reached, and we would see 36 degrees. When exactly the total advance occurs is of great importance when it comes to performance, and we discuss this in the section below on "curving."

Vacuum Advance
Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpm's, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Curving for Performance
A timing curve is simply a plot of how much ignition advance takes place over the rpm range. In other words, when the timing advances is just as critical as how much it advances.

When it comes to performance there are many different engine combinations, buildups, components, and uses….Each requiring slightly different timing curves. On the other hand if you have a stock motor, and do not care for every extra horsepower, you really do not need to do more than follow the shop manual procedures. However even a stock or mild daily driver motor can be made to accelerate faster with a five minute timing curve adjustment.

The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation. Low octane fuels ignite faster, thus require less timing advance. Conversely high octane fuel can handle slightly more advance. Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. Engines with 9.5:1 - 10.5:1 run best with 35-38 degrees total, and above 11:1, should not go higher than 35 deg. total. When using power adders such as nitrous, super or turbo chargers, the timing should be advanced accordingly.

The first step in curving a distributor is to set your initial and total advance. As detailed above and in the picture captions, the total is determined by the reluctor arm setting plus the initial advance. Ideally you should keep the initial between 10 and 20 degrees, and the total in the ranges listed above for your compression ratio. For example, if you are shooting for 40 degrees total, and your reluctor arm is on the 15L slot, you would have 30 degrees mechanical advance, requiring the initial to be set at 10 degrees.

The second step is to dial-in how fast the distributor achieves the total advance. This is controlled by the springs which hold back the weights, under the breaker plate. A stock distributor usually has one light and one heavy spring, and brings the timing in really slow, such that the distributor may only reach the total timing at very high engine speeds, 4500+ for example. For performance driving, the best acceleration comes when total advance is achieved within the range of 2000rpm to 3000 rpm. To adjust this rate, you can replace the stock springs with lighter tension springs. You can also bend the tabs on which the springs connect to change their tension.

Once you've set the initial and mechanical timing, and adjusted the curve, you should be very very close, if not right at, the optimum timing curve for wide-open throttle performance. You should use timing light at this point to confirm that the initial timing is where you set it, and steady, and then check the timing from idle to 3500 in 500rpm increments. The curve should increase a few degrees at every checkpoint until 2500-3000rpm, where it hits the maximum. After 3000 it should not go beyond the total advance.

So How The Hell Do I Know How Much Advance I Need???
Read On….

Establishing Maximum Advance Requirement
Notwithstanding the compression ratio and other factors, the characteristic that determines the maximum advance setting is the shape of the combustion chamber and the position of the spark plug.

Combustion Chamber Shape and Spark Plug Location

Combustion chambers and spark plug location and the number of plugs will have a marked effect on the time required to complete the combustion process. A large open chamber like a hemi which has a high surface to volume ratio, will combust more slowly than a wedge or modern pentroof chamber simply because it has more cold, metal molecules in contact with the combustion gasses which tends to slow reaction rates. For this reason, these chambers will require that the spark be initiated sooner to achieve PCP at the correct time.

The slowest combusting chamber would be an open chamber with a large bore size and the spark plug at one edge of the chamber. The flame front has a long distance to cover to complete combustion. By placing the plug in the center of the chamber, you halve the distance that the front needs to travel and will be able to reduce the spark advance needed to achieve maximum power. Another solution would be to add another spark plug to create two flame fronts which would also require much less time to combust. This is the solution in most aircraft engines where big bores and poor fuel distribution and homogeny require solutions to increase ignition probability.

Modern 4 valve engines with shallow pentroof chambers and a central plug location are fast, efficient combustors, requiring minimal advance for maximum power.

Most small block fords use a heart shaped combustion chamber. These require 36-38 degrees. Factor in your compression ration from above and come up with a number in the middle. These few degrees difference can be made up for with a small adjustment to the initial timing to save from modifying the reluctor arms again.

Establishing static advance requirement
The static advance requirement for a modified engine is very much dependent on the duration of the cam fitted. Below is a table of advance requirements and expected idle speeds for a range of cam specifications. ON NO ACCOUNT use these settings before the maximum advance on the distributor has been correctly limited.

When establishing static advance the golden rule is never use less than 10; never use more than 20 degrees. The engine may well tolerate more than 20 degrees at idle, but the moment the throttle is opened and cylinder filling is improved it will ping heavily. One problem often encountered when using more static advance than standard is that the engine may 'kick-back' when starting causing the starter to slow dramatically, this can be confused with a flattened battery or worn starter motor. You may need to compromise by the odd degree or two if your engine will not tolerate the required degrees of advance at start-up.

Static advance implies a measurement taken when the engine is stationery, however it is usually set at idle in order that any latency in the distributor drive gear is taken up. A rough setting can be made when the engine is still, but it MUST be set at 1300RPM or lower with the vacuum advance disconnected so that any latency is taken up and the centrifugal advance has not yet started its operation.

Establishing mechanical advance requirement
We have our desired static and maximum advance figures already calculated, so now we can use the same simple formula to establish how much centrifugal advance we need from the distributor. Example: Maximum advance 38 degrees, required static advance 18 degrees (38-18) = 20 degrees required. In our example the standard distributor is designed to give maximum advance from a starting point of say 10 degrees of static advance, if the maximum advance required is 38 degrees, then it's range is 28 degrees (38-10), this means that if the static setting is increased to 18 degrees, then the total advance will be 46 degrees (18+28), way too much. It is unlikely that the standard distributor will give the correct amount of advance, it will usually give too much. This is why we must restrict the total centrifugal advance that the distributor is capable of supplying to our new figure, in this case 20 degrees, then with the static setting of 18 degrees, the maximum advance will be 38 degrees (18+20), the correct figure.

If the advance supplied is MORE than required, and this is highly likely, it means as expected that the distributor is supplying too much mechanical advance, and that the stops in the distributor must be bent to restrict the travel of the mechanism. If the advance supplied is LESS than required which is unusual, then the distributor is supplying too little mechanical advance and the stops must be filed to allow more travel of the advance mechanism.

So what about Vacuum Advance?
Under conditions of light or closed throttle, the volumetric efficiency of an engine is quite poor, and cylinder filling is affected to the extent that the effective compression ratio is much lower than the static or calculated compression ratio. In these cir***stances the mixture will burn much more slowly than with a fully filled cylinder and the flame front will reach the piston quite late. This can dramatically cut the overall efficiency of the engine and its economy. Under these conditions the engine will tolerate and indeed benefit from advancing the timing by up to 15 degrees over its normal setting.

The device that usually performs this trick is called the vacuum advance device. The way this works is to exploit the partial vacuum that is present in the inlet manifold when the throttle is closed or partly closed. A tube is connected from the manifold to a sealed diaphragm in the distributor, which in turn is connected to the distributors base plate. The suction deflects the diaphragm which turns the base plate against the direction of rotation of the distributor thereby advancing the timing, this gives much better throttle response on part throttle, and far better economy.

Many people who tune engines disconnect the vacuum advance mechanism, and indeed on some distributors it is very hit and miss in operation and can cause anomalies in the timing. All in all however for a road engine, the vacuum advance retard should be retained if it is possible to do so (not always easy with side draught carbs). This will have a dramatic affect on economy and driveability especially on small throttle openings and when 'off-cam'.

Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpm's, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Setting Vacuum Advance.
Vacuum advance canisters are usually adjustable with a 3/32-in. allen wrench, as shown here. The small screw inside the housing adjusts the tension on the diaphragm spring. If you detect knocking and loss of power, back the screw out (counterclockwise) to decrease advance. If the engine pops and surges, tighten up the screw to increase advance.

Note:When checking initial and total advance, always disconnect the vacuum advance hose. Otherwise you will get very high timing readings.

Tuning Vacuum Advance.
The last step, after the total advance curve is set, is to dial in the vacuum advance if you have one. There should be a vacuum line connected from the carb, or the manifold, to the vacuum canister. There are two types of vacuum sources that you should be aware of. One type is known as "full" vacuum or "manifold" vacuum. This is a direct connection to the manifold, and if the hose is connected to this port you will get vacuum in the line at idle. The other port is a "timed" port, which only yields a vacuum above a certain rpm. At idle the line will have no vacuum. Most carburetors have both ports. On Holley's the timed is above the throttle blades, and the "full" is below, near the base. On Carter/Edelbrock carbs, the timed port is on the passenger side and the full is on the driver's side. The easiest way to confirm what port you have is to hook up a vacuum a gauge and check for vacuum at idle. The preferred vacuum source is the timed source. This way there is no effect on the initial timing setting.

Remember vacuum advance is load dependent, so you cannot check it with a timing light with the car in neutral. The best way to set vacuum advance is by feel, under real driving conditions. Connect the vacuum line and drive the car up a steep, long grade, with the car in high gear and at a low speed, 30-40mph. Occasionally push the accelerator to the floor, and listen and feel for knocking and/or loss of power. If you detect this, immediately back of. This means the canister is advancing too much and you should adjust the canister so the diaphragm is 'tighter', by turning the screw counterclockwise.

You can also adjust the vacuum canister clockwise until it does start to ping and then back it off 2 turns. This should set your Vacuum Advance perfectly.