Original submission by T:
 Ignition:All Voltages displayed in this page are measured with respect to the chassis. In other words connect the black lead of the voltmeter to the chassis.
 Types of Ignitions:While the ignitions fitted to old Holdens were traditionally the equal or better of many international cars at the time, these ignitions pale in efficiency compared to later ignition units. Tests have shown that increasing the spark energy that goes into the cylinder increases the power the engine will produce and the better the efficiency of the engine will become. Exhaust emissions will reduce as well.
Aero engines carry dual ignition systems that spark two plugs in each cylinder. Turning either magneto off causes the engine RPM to drop by up to 10% and there is a noticable loss in power.
 The Kettering Ignition System:The original ignition systems fitted to old Holdens used Kettering Ignition (named after Charles F Kettering, the founder of DELCO, the Detroit Electric Laboratory Company and inventor of that ignition).
 UnBallasted Ignition Coil. Kettering Ignition:
Consider this circuit to be powered by a perfect battery.
Holdens that were equipped with Grey motors had no Ballast Resitor in them. When the points close the current through the points and coil steadily increase. Energy is accumulated and stored in the form of magnetism between the ends of the iron core of the coil. When the points open the magnetic flux is converted into electricity in the Secondary Winding which results in a spark.
 Ballasted Ignition Coil:Consider this circuit to be powered by a perfect battery.
This circuit uses a Ballast Resistor in series with the Ignition Coil. In Holdens, from red six powered cars onwards, the Ballast Resistor was incorporated into the wiring harness and was a pink coloured wire, printed on it was "Resistance - Do Not Cut"....on every Holden wiring harness I've repaired this wire has melted through in one spot or more, and I suggested to the customer either Electronic Ignition or an external ceramic ballast resistor, If the harness was removed from the car I would remove the wire, otherwise I would just cut it at each end and leave it. Its not a very smart design, running a wire that WILL generate heat in a harness, and It isnt even fused. Holdens that were equipped with Grey motors had no Ballast Resitor in them. At Idle the Ballast Resistor heats up and prevents the Coil from overheating because the Coil has more time to conduct current. As the RPM increases the Ballast Resistor cools down and allows more current through to the Coil to compensate for the reduced time available to charge the Coil. When a single pair of Breaker-Points is used the system needs a 50% duty cycle. i.e the points close for the same amount of time that they are open. With these points there is little lattitude with this and a change in Duty Cycle will result in diminished Coil output.
 Dual Point Ignition:Consider this circuit to be powered by a perfect battery.
 L34 Dual Coil Distributor Animation:
These early Breaker-Point distributors suffered from Points-Bounce. That is the harder the points slammed closed, the more they tended to bounce open again like a rubber ball bouncing on concrete. When the points bounced they caused the Coil to spark prematurely causing one of the plugs to spark at the wrong time. The bounce also prevented the Coil from generating enough energy for the next spark. Points-Bounce affected the note of the engine. Distributors that are Magnetically or Light-Source triggered cause each spark to occur at exactly the right time every time. As a result the engine note is one of precise firing of each cylinder as opposed to a general sort of "rasp" that the early engines fitted with Breaker-Point ignition made.
 Transistor Switched Ignition:This system is the normal Kettering System ignition but with the Coil Primary Winding switched by a High Voltage Power Transistor. The Breaker-Points can be retained to trigger the electronics or a Light Source or Magnetic Trigger can be used. A Points-Bounce circuit limits reduces false triggering. Even though this system can use the same coil as the Kettering System, its output is higher because the Power Transistor switches faster. The faster the voltage in the Primary Winding changes the better the efficiency of the coil, just as an Alternator increases its output with an increase in RPM.
 Dwell-Extended Transistor Switched Ignition:This system is identical to Transistor Switched Ignition except that the Power Transistor holds the Coil Primary current at a maximum until a signal from the Breaker-Points, Light-Source or Magnetic trigger appears. This system uses the same Coil as the original Breaker-Point Kettering System but produces more Spark Energy because the Coil Primary is both switched by a Power Transistor and because the Coil Primary is given a much longer period to saturate the Coil's former. The use of electronics permits a big change in the duty cycle which can be extended from 50% on time 50% off time to 90% on time or more because only a brief time is needed for discharging the coil.
 Capacitive Discharge Ignition:With the advent of transistors came CDI. CDI produces an intense spark of very short duration. Typical measured figures for an MSD6 CDI for example are 250 milliamps for about 0.3 milliseconds compared with 60 milliamps for about 1.5 to 2 milliseconds for a HEI ignition. Because of its brief exposure to the mixture around each plug a CDI needs a wider spark gap to take effect. As the mixture is compressed it is churning around greatly and passing through the plug gap. A long duration spark will light more fuel+air blowing through the gap than a very short spark so there will be a better chance that the flame finally spreads through all the mixture in the time available. A High Voltage Inverter contained within the CDI system charges a capacitor bank with hundreds of volts (typically 450v). When the ignition signalling device triggers it, the CDI unit dumps those hundreds of stored volts across the coil primary. The secondary coil output is intense in energy but short in duration. CDI ignitions, because of their very fast spark voltage rise-time, can sometimes have problems with "crossfiring" if one plug wire is routed in close proximity to another, it can cause the second plug to fire as well. Engine damage can result. CDI ignitions are used on 2-stroke motorcycles and outboards where their ability to fire a fouled plug (a common problem with 2-strokes) is of more value than the problems introduced by their short spark duration. Crane Hi-8 is a CDI system. MSD make IDI (Inductive Discharge Ignition) and CDI ignitions.
 High Energy Ignition:The HEI system uses a special Coil and Electronic Ignition Module combination. You must use a Coil nominated by the manufacturer otherwise undesirable outcomes can persist. High Energy Coils cannot sustain direct and full battery voltage across them. If they are left connected directly across the car's 12 volt battery the Oil-Filled version of the Coil will blow its safety valve and spurt hot oil everywhere. The Coil will eventually fail. The plastic C-Core version of the Coil (the Bosch HEC716) will explode. It is the job of the Electronic Ignition Module to ensure the long life of the Coil by only applying the full Battery voltage to it for a brief period. The module also limits the current passing through the Coil's Primary Winding to a sustainable level. The Electronic Ignition Module varies the on time and maximum current passing through the Coil's Primary Winding consistent with Engine RPM. Though HEI systems are known to suffer at high RPM, they are excellent for street and highway use. The Electronic Ignition Module uses a High Voltage internal Power Transistor to switch the current in the coil primary. The concept is like connecting a Six Volt Ignition Coil directly to a 12 volt battery for a very brief period. Consequently the spark energy developed is higher due to both the power applied to the coil and the fact that the Coil's Primary voltage rises faster (more volts per second). This improves the coil's efficiency, known as dv/dt switching. Like CDI the duration of the output spark is brief so a wider plug gap must be provided to expose more of the mixture to it. In distributor based Holden HEI's, the spark-plug gap is 1.5 mm.
 HEI Tips:It is vital that the white Silicon Grease be placed between the underside of the Electronic Ignition Module and the alloy surface it mounts on. This grease improves the heat transfer from the module to the metal surface that it is attached to. If you don't do this the Module will overheat, begin to perform intermittently, then ultimately fail. Periodicallly check that the 2 screws that secure the Electronic Ignition Module to the distributor haven't worked loose. The HEI Coil is a greedy device. It expects to be fed current through a 30 amp cable. In reality it uses a maximum of 6 amps peak, but using nice heavy wiring to supply it guarantees it will always be fed with full battery voltage no matter what. This will benefit high rpm operation in particular. The Vacuum Advance Module is easily removed and replaced with the Distributor complete and still in the car. There is no clip or fastener that holds the Vacuum Advance Unit Shaft in place. There are only the 2 screws on the side of the Distributor Housing. The Vacuum Advance Unit Shaft has a hole drilled in it that fits over a Dowell in the underside of the Outer Ring of the Magnetic Pickup. You are removing the Shaft from the Dowell. Just persist with these steps until it comes free. Remove the Vacuum Advance Line, then the 2 screws that secure the Vacuum Advance Module to the Distributor Housing. Keeping it in its natural plane, gently pull the Vacuum Advance Unit all the way out. Take a flat screw driver and push down on the Shaft of the Vacuum Advance Unit. If it still won't release wiggle the Vacuum Advance Unit right to left about its axis as you push down on the Shaft with the screwdriver. Replacing the Vacuum Advance Unit is done by rotating the outer ring of the Magnetic Pickup fully in the anti-clockwise direction. Slide the Vacuum Advance Unit all the way into the Distributor Housing along its alignment. Hold the Vacuum Advance Unit Shaft up against the underside of the Outer Ring of the Magnetic Pickup and pull it slowly backwards until you feel the hole snag on the Dowell. Wiggle the Vacuum Advance Unit from right to left to work the hole onto the Dowell. If you think you have it connected properly you can test it by pulling the Vacuum Advance Module in and out of the Distributor. The Outer Ring should now be coupled directly to the Shaft and rotate around as you pull the Vacuum Advance Unit in and out. When you are sure the dowell has engaged the hole, screw the Vacuum Advance Unit onto the Distributor Housing with the 2 retaining screws and reconnect the Vacuum Advance Line.
 General Ignition Tips:Over time the High Voltage applied to the Plugs, Leads, Rotor Button and Distributor Cap will cause them to break down. Periodically replacing the lot will save you hours of troubleshooting mysterious problems that disappear the moment you open the bonnet up. There is a small Carbon Brush in the centre of the Distibutor that accepts current from Coil and connects it to the Rotor Button. This brush will eventuall wear out making it harder for the Coil's current to get to the plugs. The Distributor Cap can develop carbon tracks underneath the cap and make it easy for the Spark to jump to ground or the wrong place rather than be directed to the right Spark Plug. The Mechanical Advance Balance Weights need oiling every tuneup. Remove the Rotor Button and put 3 drops of engine oil onto the pad under there. This will keep the balance shaft lubricated, stop it from wearing out and help to keep the advance plot in shape. Over time the rubber hoses that feed the Vacuum Advance will begin to perish. When first fitting always make these hoses longer than they need to be. The ends of the hose are the biggest problem because they plug into hot brass fittings. This makes the rubber go hard and lose its ability to seal properly. When this happens cut one inch of the end of the hose and push the new end onto the brass fitting. By the time the hose becomes too short the whole hose will be ready for replacement. Spark Plug leads should be twisted off, not pulled. They come off most easily with a gentle twist and that will prevent the internal conductor from being stretched or stressed.
 High Compression Ratios, Detonation, Pre-Ignition and Advance:My Pre-Ignition and Detonation experiences have been interesting, so if you don't mind I'll give you the full answer. There's a general policy to back off the advance to stop or mimimise Pre-Ignition and Detonation and I've found maximising advance provides the best answer. All the Blue Sixes pull on the same degree of Mechanical Advance, they just use heavier or lighter springs to invoke that advance sooner or later. I use the lightest balance weight springs. Bosch can advise on these. The EFI VK (Black Motor) Distributor pulls on only half the mechanical advance of the Blue Distributors, which is why the static setting is 12 degrees as opposed to the normal 6 degrees. The Vacuum Advance Module I use has 230 stamped on it. The 2 is the diaghram's internal spring tension and the 30 equates to 3.0mm of travel from relaxed to fully activated. The degree of travel determines the amount of advance pulled on by the module. The bigger the number, the further the module pull rod travels under full vacuum and the further the HEI stator ring gets pulled around resulting in more advance. The standard Blue Distributor Vacuum advance will have 212 or 214 stamped on it. That means a spring tension or 2 and 1.2mm or 1.4mm of travel. These numbers are pretty tame compared to what the engine can take. When you look at the Advance Module, you'll see that the 1.2mm relates to the length of a nick cut into the Module's pull rod. A 214 will have a longer nick and a 230 will have an even longer nick. I believe it may be possible to file a Module's movement nick wider to make it travel further. It's something I've never tried but I could believe it would achieve the same result. I've also seen modules with 165 stamped on them which may be an imperial measurement, but those also had a heavy spring and very limited travel. If a distributor has one of these I'd expect heavy detonation from the engine. The GM Part number for the 230 Vacuum Advance Module is VS 141414, that is if they are still available, but a recent poster claimed that Bosch HEI Distributors can be bought brand new for around $250. The VS 141414 Module produced a pleasant increase in performance and economy. A rumour had it that this module was the standard V8 module. There will always be a lot of debate about pre-ignition and detonation since there are many variables involved. My experience has been that combustion chamber deposits are the biggest cause and that running the engine at peak efficiency will burn the combustion chamber clean and deliver power free of detonation and pre-ignition. I am a great believer in GM wedge combustion chambers because they can accept much more advance than other combustion chamber designs. I've found Alfa Romeo hemi's to be particularly susceptible. Detonation came in heavily when GMH were forced to detune their engines to meet emission requirements, i.e around HX. My HZ in standard form used to detonate and pre-ignite for fully 30% of any Sydney-Canberra-Sydney run I did. The issue could be managed by flooring the accelerator, backing off, or partially activating the choke. After increasing the spark energy and the spark advance in steps, the pre-ignition and detonation vanished so I've operated on the basis of keeping maximum advance and spark energy ever since. EGR pays detonation control dividends too. I've run my engines with and without it and found that running with it delivers the best power and economy. The thing I've found with detonation and pre-ignition is not to run away from it. The weaker the ignition and the fewer degrees of advance the more combustion chamber deposits form and the worse the detonation will be. I use Caltex Vortex. In addition to being the right octane rating, it has washed years of accumulated EGR ash out of the inlet manfifold and delivered on all their promises over the 9 months I've used it.
 Ignition in the 2-Stroke Environment:... yours is a good question because the specific environment you're working in has specific needs. The short answer is that 2-Stroke technology thrives on CDI type ignitions. This is because CDI has the ability to spark fouled plugs, a big advantage in an environment with lots of contaminants like an excess of lubricant and the heavy Exhaust Gas Recirculation the 2-Stroke expansion chambers provide. It's clear that 2-Stroke technology was into EGR long before any 4-Stroke and that the process of using the 2-Stroke engine's own exhaust to effect natural supercharging precipitated the high power outputs of Kawasaki 750 triple motorcycles. Robbie Coltrane's Planes and Automobiles has a great expose on CZ motorcyles when expansion chambers were first exploited to full advantage. While the common perception of 2 Stroke "missfire" is the collision of the exhaust gases with intake gases, I don't agree. It's little different from valve overlap in a 4 Stroke. I think the real reason for 2 Stroke "missfire" is weak ignition. The same reason early 4 stroke cars used to gag and stall. If you watch 12 O'Clock High, you'll see the '37 Ford gag as Gregory Peck drives away from the sentry point just before he enters to take over command of his old post. Our old friend weak ignition at work. A closer look at the expansion chamber process reveals that the last charge will be forced back into the 2 Stroke Engine's cylinder the instant before the piston rises. This occurs as a result of the sonic tuning of the expansion chamber to match the timing of the piston's frequency. Radio engineers would refer to the "Q Factor" of this phenomenon to describe the range of piston speeds for which the expansion chamber will be effective. Many are aware of the "power band" effect that used to be common to motorcycles that had peaky expansion chambers. Essentially the expansion chamber will continue to pump an ever increasing charge back into the cylinder until something has to give. If the engine manufacturer keeps the spark weak the engine will eventually missfire because the coil cannot generate sufficient energy to push a spark across the spark plug. After the missfire the expansion chamber pressure begins to ramp down its pressure until the ignition system can cope again which is when the engine recommences firing. I note that a 2 Stroke running the critical load tends to "missfire" less. I put it down to the fact that the expansion chamber can no longer build up runaway pressure, and so the ignition system can continue to fire it. The point of sharing all this info with you is to give the back ground on why I think CDI is best, or that HEI would be the better choice over any medium strength ignition system (MEC). As previously posted, 4 Stroke engines do benefit from long spark durations. This is because in their standard normally aspirated form, their compression pressures are predictable and don't run away courtesy of positively timed valves. God knows they need it though since they are only firing every other stroke they need all the efficiency they can get. In the trade off stakes a 4 Stroke engine can deliver power OK from a weak ignition if the spark has a long duration. This can easily be achieved by using a physically large coil which won't necessarily produce a higher voltage but which can sustain a long duration of output because there is so much more magnetic flux to dissipate, in much the same way that a larger battery takes longer to discharge. Given that cars mostly have larger generating/battery systems than most petrol vehicles (except for aeroplanes that use magnetos) the practical ignition source for 2 Strokes served by an alternator with a large charging system like a big outboard would be HEI because of the runaway efficiency tendency of the current trend to run expansion chambers. Once again I would nominate CDI, but if your charging/battery system can sustain the 7 amps that the Bosch HEC 716 HEI coil needs then you will do well provided you remain aware that HEI does experience a drop off in energy with frequency. If your total ignition needs are no more than for a 6 cylinder 4-Stroke engine running at about 6,000 RPM, you'll be alright. Otherwise you'll be forced to use CDI since CDI will still be delivering murderous voltages up around the higher RPM. To extend the background further, the best 2 Strokes (so I've read) are big bore short stroke designs. This works best for port exposure and crankcase scavenging. In the least it suggests that the natural tendency for an over square engine to rev (some would say that it needs to) there is less time for the charge to be exposed to the spark. HEI is a clever integration of the high voltage of CDI with the long duration of IDI. The trick is that you need something like the capacity of a car battery to make it work properly. As Capacitive Discharge Ignition overcharges a standard GT40(R) type coil with 300 volts but produces only a very short duration and the same coil operated in a different mode can have a comparatively very long duration HEI provides the best of both worlds by reducing the activation voltage from 2,000 % down to only 200%. The resultant output is CDI type voltage with Inductive Discharge Ignition (Kettering/breaker poiint style) type duration. Also of note is the situation inside the cylinder after the combustion process is initiated. For too long the world thought that initiating the flame front was all that was necessary, whereas NASA proved that if you can continue pouring spark energy into the cylinder, the power output increases. Early ignition systems suffered from "Discharge Lag" in that when flame front caused the pressure inside the cylinder to skyrocket well above any level the coil could cope with, the coil's energy just dribbled away. With CDI or HEI, the ignition system has sufficient reserve to keep pouring in spark after the flame front has been initiated with obvious improvements in power output. To sum up, the normally aspirated 2-Stroke environment will always need higher voltage than the normally aspirated 4-Stroke, so CDI/HEI is a must for efficient 2 Stroke operation and HEI over any MEC ignition/coil set.
 Waste Spark:One big factor in detonation and pre-ignition is combustion chamber deposits. These are most easily formed when the engine has weak ignition, insufficient spark advance and the engine doesn't see enough load. If the above items are properly set then combustion chamber deposits will diminish and so will detonation and pre-ignition. Light loads with weak ignition and poor advance also causes the piston ring grooves to fill up with carbon. The piston rings will then no longer gas pressurise. This causes a loss of power in a 4-Stroke and backfiring in a 2-Stroke when the flame front blasts past the piston rings and ignites the crankcase charge. In both engines, steaming them will cure it and proper running keep it away. Many engines are rebuilt or thrown out because it is thought that the piston, rings and bore are worn out. In actual fact those three items can be quite sloppy and not cause a problem. I've operated aero engines up to 540 CID. They have been very time (over their 2000 hour Time Between Overhaul) by at least 10% (with a government dispensation to permit continued operation), so much so that they could be pulled through compression by a finger tip near the propellor hub. Once started and with the compression rings gas pressurised those same engines stuffed out the same power as relatively low time engines. You're probably aware that detonation is when the ignition occurs at the correct time but burns too rapidly. Pre-ignition is when combustion is initiated prior to the spark occurring. Glowing deposits are a usual cause of the trouble and the trouble is often mitigated by proper use of the engine in addition to having any weak ignition or inappropriate spark setting cured. Proper use would be 75% cruise power for a sustained period. Improper use would be extensive trolling. I'm not familiar with the term "waste spark". It wouldn't surprise me if there were some very sophisticated ignition systems around for outboards. Last I looked, one model Chrysler 2-Stroke outboard was the world's cleanest running internal combustion engine. It is my view that you cannot give an engine enough spark. The more it gets the better it will run in every possible way. Turn off 1 of 2 magnetos in an aero engine and the RPM drops noticably even though both magnetos are perfect and timed to spark at exactly the same time. I did build the Electronics Australia CDI for myself and a number of people for cars and motorcycles. I found it superb in operation, very easy to build and the individual components were dirt cheap. The power transistors were 2N3055's. I used to run my CDI from 2 flat Dolphin batteries under cranking. Once the engine fired I switched over to the car's battery/generator. CDI draws much less current then HEI, but it does need something as big as a pair of flat Dolphin batteries to get the inverter started. It sounds like your 15 amp alternator will cover HEI. I just researched "waste spark" on the web. Though I hadn't heard the term before it's a concept I invented some years back, just like Fan Braking. The important distinction here is in this case "Waste Spark" means "Sparking the Waste" and not "Wasting a Spark". Yes, I'm a big believer in it. I think this is what Multi Spark Discharge ignitions are onto but the difference it to spark all the cylinders that are still on their power stroke and not just the current cylinder. I designed a rotor button that kept spark pouring into the last 3 fired cylinders to achieve that result. Waste spark in a 2-Stroke can only be advantageous. The cutoff point for each cylinder should ideally occur before crankcase scavenging. I think the advantage would be just as great for a 2-Stroke as a 4-Stroke though exponentially better in the 2-Stroke case.
 Coils From Positive Earthed Cars:Coils from some Brit cars can look like ordinary negative earth coils but are actually wired for a positive earth. Positive earth coils will soon blow the capacitor and the car will run increasingly badly as the plugs warm up. They will produce very high primary voltages because the ground point for the secondary winding is connected to the points connection rather than the supply side. These high primary voltages are more than the capacitor can endure. Also a hot spark plug allows the spark to jump more easily from the centre electrode to ground. The plug behaves like a radio valve and donates electrons easily. When the spark voltage tries to jump from a cooler ground electrode to a hot centre electrode electrons are not donated so easily. As a result more voltage is required to bridge the gap in the reverse direction. The way to identify if a coil is correctly polarised is to place the exposed lead part of an insulated pencil between a plug lead and a plug. The correct result should be a flare of lead that flashes towards the plug. A bad result will be that the flash will shoot back towards the coil. This test is not valid for double ended coils.
 General Ignition Facts:Weak ignition needs a rich mixture to fire. A Coil that can spark in air may not be able to spark the Spark Plug under compression. Colder air needs more spark. Higher compression engines need more spark. The Spark Plug gaps burn wider with use. Wide Spark Plug gaps need more energy to spark them. Less spark is needed at Idle than under power. The traditional time for faulty Ignition to missfire is on opening the throttle from Idle. After fitting a new Engine a fault in the Ignition is likely to show up because the compression will be higher than on the old engine. The Coil may fail.
 Setting Dual Point Ignitions:
 How To Judge a Good Ignition System:In the circumstances where an ignition manufacturer's claims are in doubt a proven wider than standard plug gap coupled with an across the board improvement in running, is a basis for a good system. Improved starting plus the ability to stab the throttle wide open under very low RPM without the engine gagging is an indication that the ignition can deliver the highest voltage under the greatest cylinder pressure demands of a normally aspirated engine. It should also deliver superbly under peak RPM without the need to close down the plug gaps or richen the mixture which gives an indication that the ignition's frequency response is adequate for the RPM you want to run your engine at. Turbo/supercharged applications will have even greater requirements because their Ignitions will have to deliver both higher voltage and high frequency response at high RPM. Be aware that a significant increase in spark energy can require improved insulation in the Spark Plug leads and Distributor Cap, Rotor Button and other components. Mostly the Ignition demands of an 8 cylinder Engine will be higher than those of a 6 cylinder Engine and a 6 Cylinder Engine's Ignition demands will be higher than those of a 4 Cylinder Engine because the more sparks to be delivered per Engine revolution the harder the Ignition has to work. Typically a V8 will encounter the frequency limitations of an Ignition System before the engines with fewer cylinders.
 Ignition Troubleshooting:
 Coil, Capacitor, Points, Wiring Test:
Coil, Capacitor, Points, Wiring Test
Submitted by T on Sun, 03/06/2007 - 07:13.
On HZ's, the harness near the Wiper Motor can become disconnected and also burn. Check to see that good connectivity is being made here before proceeding with the next steps. Remove the dizzy cap and rotor. Pull the points open and place a piece of paper between the points so they can't contact each other. Remove the coil lead from the centre of the dizzy and tie it to the engine someplace with a small gap so you can see it spark. Make a direct connection from the battery positive to the coil positive. Flash the coil negative to ground with a separate piece of wire. If the coil secondary doesn't spark the coil or capacitor (condenser) is dead. Unscrew the capacitor and isolate its lead so that it cannot touch anything. Flash the coil negative to ground. Still no spark, the coil has died. If a spark does appear then the capacitor is faulty. If you get a spark with the coil and capacitor connected, then remove the battery wire and rotate the engine until the points close. If the points won't close, there's your trouble. Readjust the points for correct gap/dwell. Reconnect the battery wire to the coil positive and open and close the points with an insulated screwdriver. If no spark, the points have had it. If you have a spark, put the ignition back together but leave the battery wire connected. Crank the engine. If the engine starts OK, then the wiring to the coil has died. Run new wiring or re-terminate. If the engine doesn't start, are the points opening? If not, there's you're trouble. To kill the engine, short the coil negative to ground or remove the battery feed to the coil positive.
End of submission by T.
 A Coil Test:
Posthumously added for Jacks Nov 9 2006:
"The only way I know of testing a coil! Is to put a simple test light. (Not a multimeter)With the clip connected to the positive side of the coil and the test light touching the negative side of the coil, with the distributor cap off, but all other wires connected and the points closed! When the ignition switch is activated, via a key or push button setup. It should flash intermittently! Showing that it is working properly! Now, holding the points open with a piece of cardboard, if it shows continuous light that is an indication that the condenser or points wires are earthing to the distributor outer casing! If there is no light at all, your coil is stuffed!"
 The Engine Cranks But Doesn't Fire:
If an engine won't fire and after rotating the distributor sufficiently in the advanced direction the engine does fire, it's a guaranteed sign of weak ignition. If you keep moving the ignition timing in the advanced direction (anti-clockwise in a red/blue/black six), the ignition will see a lower cylinder compression in the current cylinder and find it easier to spark the current plug. There will then be a fire of some sort in the current cylinder. If you've advanced the timing back far enough at this point, the inlet valve will be partially open and the engine will backfire through the carby. Although the process of fiddling with the timing is unadvisable, it is the fastest way of establishing that the engine is not starting due to weak ignition when you have little else to work with (no multimeter or replacement parts etc). Don't forget that the ignition can spark in air and look OK, but under compression is when the greatest igntion load is present. Wide open throttle at cranking is when standard engines see the greatest compression pressure. I have seen an HEI coil that could throw a spark longer than 1" into free air but failed completely under compression. The fast confirmation was to put the dizzy in so that the engine fired on valve overlap for that cylinder, (either that or relocate the plug leads in the distributor cap). On the next crank, the engine backfired through the carby. The next step was to replace the coil which fixed the problem. Under inspection the original coil had a vibration fracture at its mount point which caused to break down under peak voltages. Be advised that anytime the spark is over-advanced it might result in a carby fire, especially if the aircleaner has been removed. Among other things it's the job of the aircleaner to act like a Davy Safety Lamp and trap the flame under it. So take every possible precaution. Keep your eyes away from the carburettor since any flame or blown back carbon can cause personal injury or even loss of sight.
 Ignition and CD/SU Carburettors:Constant Depression carburettors such as S.U's (Skinners Union) keep the ignition demands down i.e. the slides open up gradually and only as the ignition is able to respond giving the combustion time to develope. A carburettor like a Holley, or any other non-CD style carburettor, demands strong ignition everytime the throttle is opened and if the car has weak ignition it would guarantee a gag and flat spot. Thinking further about SU/CD carburettors I got to thinking about Brit cars, their traditionally weak ignitions and SU/CD style carburettors, it might explain why this style of carburettor was heralded over there on certain cars, whereas others have not found the same success in different regions with different makes of car. Those vehicles that came standard with Lucas "prince of darkness" ignition would go better with any kind of Constant Depression carburettor since the ignition demands are reduced. Fitting the same CD carburettor to a car with powerful ignition might have left others wondering what the fuss was all about. A further thought is that SU/CD carburettors were developed during a period of notoriously weak ignitions and as a great man once said "90% of all carburettor problems are ignition". This means that the entire SU/CD concept may have appeared in response to weak ignition either masking the problem or persisting in the face of it with a sort of "look I found the solution" response when in fact it was just a workaround.
 Cold Weather:Ignition faults appear with bad weather because the increase in air pressure requires an increase in spark energy.