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Why does it work?

The other day I stopped to help a young man and his wife who were stranded in an empty parking lot. They had run their battery down trying to get the engine started in a late 60’s Camaro. I quickly found the problem, it was a broken wire to the coil and I did a quick repair. I hooked up jumper cables and let their battery charge. The engine was flooded because they had been pumping the accelerator like crazy trying to get the engine to start. When the battery was charged enough, I pulled up the coil wire and let it stay just near the top of the coil. As soon as the starter turned over the engine started with a plume of grayish smoke from both tailpipes. I had him shut off the engine and replaced the coil wire. He tried it again and the engine fired right back up. My dad taught me this trick many years ago and it has always worked for me. I would just like to know why it works even on badly flooded engines. I once pulled out a spark plug and cranked it with the coil wire out and the spark plug looked like an arc welder because it was so bright. My dad said that he learned this trick from an old mechanic buddy in his army days. He also said it could burn out something so it was only used when engines would not start due to flooding. This experience got me to thinking…why does this trick work?

When the coil wire is pulled up and a gap is put in the secondary circuit the secondary voltage jumps to enable it to overcome the added resistance. The increased voltage more readily jumps the gap at the spark plug and even a few sparks from random plugs will allow the engine to begin to run, albeit roughly, but quickly blow off the wet electrodes and soon run normally.

Edited, I’ll have to figure that response out. Holding the gas pedal to the floor and cranking keeps the choke open and allows more air to flow, or forcing the choke to stay open as my fix for a flooded carburated engine

I’ll argue that all you really did was repair the coil wire and allow the excess gas to blow out when the cylinders fired. With respect to Rod, the total peak voltage out of the distributor is simply a function of the voltage at the primary, the number of coils that collapse the current into the core, and the characteristics of the coil and its core less resistances of the points. The condensor is irrelevant because although it’s a capacitive device and stores voltage it can only charge to the total of the peak voltage applied. It cannot add voltage.

Actually, that gap you created by holding the ignition wire off the distributor cap acted as a resistive item in the secondary and “dropped” some voltage (converted it to heat energy in the arc), thereby reducing the voltage that the spark plug saw.

I’m open to competing theories.

The coil builds voltage in the secondary until it gets high enough to overcome the impedance of the circuit (electrode gap, fuel/air mixture, pressure).

Fuel wetting the plug essentially reduces the impedance of the plug and so the coil will not build up much secondary voltage before it arcs weakly and dissipates the charge on the coil. When you pull the coil wire, you re-introduce an air gap with high impedance and the coil voltage can now build up to a sufficient level to produce a strong arc that can bridge both the coil wire to coil gap you added and the wet spark plug electrodes. The problem can occur when the secondary voltage builds to a high enough potential to start frying ignition parts not designed to withstand the higher voltage. You have to be careful with the size of gap you introduce or the voltage can get dangerously high.

On newer electronic ignitions, there is increased risk to the relatively sensitive integrated circuits used in those designs as compared to the older, points style ignitions.

My original response was a spark with less oomph should not start it better, but trying to think about Rod’s explanation

If I try to understand this the rotor may not fire to a couple of plugs increasing the coils charge until it finally can jump the gap, then creates a bigger spark?

The voltage spike out of the secondary is a very sharp spike to maximum when the points open and allow collapse of the coil field, said voltage hopefully sufficient to jump the gap.

Said differently, the voltage does not build until it’s sufficient to jump the spark plug gap. It spikes at collapse and ramps down rapidly with smaller spikes riding on it from the inductice process. The process happens when the points open and break the primary circuit. It cannot exceed the max induced by the coil windings.

I totally agree with that method. However, the engine did not even try to start with the gas pedal down. I even took off the breather and that did not help either. I went to my quick fix even though I don’t like to use it. I know I was taking a chance of frying something but the owner said go ahead. The engine does run terribly but it clears out the gas. He had a large carb on the 327 (1150 double pumper I think) and there was a lot of gas in the intake when I arrived. I told him a 650 would do the job but he disagreed.

Thanks for the response. I think I’m beginning to understand what’s happening when the coil wire is removed. The secret is in the “gap” created? I know that when the vehicle starts there is a strong blue arc jumping from the coil wire down into the coil and it actually is so strong that it sometimes jumps to the nearby terminals on the coil. There is also a loud “buzzing” sound that is clearly heard above the engine. I know the ignition system is not strong enough to handle that sort of voltage so I always turn the engine off quickly.

TwinTurbo’s explanation is exactly how I understood this to work, as well as why there was a higher voltage spark at the plug when the coil wire was pulled back a bit.

I too remember this technique from mechanics I used to work with - though I never really used it in practice.

Let me tell you the real reason this may work. It has to do with something called the LR time constant. Simply stated, it is the time it takes a coil to charge or discharge based on the resistance of the charge/discharge path. The lower the resistance, the faster the charge/discharge.

Coils resist any change in current flow. If a voltage is applied to a coil, it takes a while (LR time constant) for the current to increase to where Ohms law says it should be. Now if you are familiar with how transformers work, the output voltage is determined by the input voltage times the ratio between the number of turns in the primary and the secondary. Transformers work with AC as the source or supplied voltage. The LR time constants are the same for both the applied and the output voltages.

Ignition coils are different. While they have a primary and secondary windings with about a 100:1 ratio, they do not work like a transformer because of the different LR time constants between the applied and the output. Direct current is applied to the primary, building up a magnetic field. When the source is suddenly switched out, the coil wants to keep the same amount of current flowing. It takes energy from the magnetic field to do that. The coil will develop what ever voltage it needs to keep the current flowing.

Increasing the gap at the coil does not deliver any more voltage to the plug, but it does increase the total voltage developed by the coil. The additional voltage is dropped by the second gap you created. But it also increases the duration of the discharge, and that is what helps the engine start quicker.

This is OK for older point type (Kettering) ignition systems. I would not recommend doing that to a modern vehicle.

BTW, the reason why the coil has a primary and a secondary is to protect the switching network. Most people think the coil is acting as a step up transformer, but its not. During discharge, it is acting as a step down transformer. The voltage developed to jump the spark plug gap would destroy the switching network. The 100:1 ratio steps down this voltage by 100 times to protect the switch.

It’s a TIME thing…The wet plugs allow the secondary voltage to bleed off to ground WITH NO SPARK across the electrode gap…As the limited amount of currant is bled off through the wet insulator, the voltage never rises high enough to jump the gap. By introducing a second air-gap in the secondary circuit, the voltage must rise enough to jump THAT gap, and in doing so can easily jump the across the wet plug. That’s why 2-stroke engines all went to CDI…The capacitor discharges INSTANTLY into the coil and the voltage rise is so fast, there is not enough TIME for the current to bleed off before the plug can fire…

Points do not open instantly. It takes TIME for current flow to stop and allow the field in the coil to collapse, producing the spark…If currant can flow before the voltage climbs high enough to jump the plug gap, the plug may never fire…A fouled (wet) plug allows this current flow…The secondary gap you introduced blocks the current flow and allows the voltage to rise high enough to jump the gap…

Caddyman, stick to what you know, I have 45 years of experience in electronics and not enough space here to explain things like Ohms Law, inductive kick and voltage dividers to you. Simply put, voltage developed across one gap does not transfer to the next gap. Current is constant throughout series circuit, but the voltage divides across each resistance.

Excellent post, Keith. You cleared up what had always been a mystery to me: Ohm’s law says that in a series circuit voltage progressively drops across the resistance portions of the circuit, but that at any given moment amperage is the same everywhere.

So I could never understand how, by adding resistance at places in the secondary circuit, you would end up with more available voltage at the air gap. But I assumed wrong–you get more overall available voltage by adding resistance.

Also, with the flooded condition, you would have a really, really abnormal rich mixture, and since the spark doesn’t jump an air gap (it jumps an air/fuel gap), and since the richer the air/fuel gap, the lower the resistance, there had to have been abnormally low resistance in the circuit. Given this, it would make sense for missleman to create some more resistance by pulling the coil wire out of the coil a little.

You said that the increased overall resistance in the secondary circuit increases the duration of the discharge, in other words, the spark duration is longer, right? But it must also make the spark cooler, no? But I guess with the fuel-saturated condition of the combustion chamber, spark duration would be more important than spark temperature.

The pieces of the puzzle are starting to come together now, I think!

I working on an angle of the coil being forced into full saturation by this gap provided. I think it is clear that a fully saturated coil is what is desired. Typicaly coil saturation time is dependant on the resistance of the coil primairy winding and the voltage applied to it. I wonder if this coil wire trick worked because it compensated for a defect in the switching circuit or the coil it self. The "full saturation’ idea is pretty close to Rod’s inital idea

My text gives the technical name of “pulse transformer” to an ignition coil, if anyone wishes to explore ignition coils.

I could buy the saturated coil theory if the additional field generated were sufficient to create enough addition current that upon collapse the induced spike were sufficiently larger to offset the voltage drop at the gap created by holding the secondary wire “just near the top of the coil”. But I don’t believe it is. I’d have to see the actual spikes to buy into the theory. My money says the loss from the additional gap would more than offset anything gained by allowing coil saturation.

I like this thread though. It’s a good workout.

I read Keith’s reply carefully and after thinking it through, I have to admit my earlier reply was wrong. He’s correct. Not to mention that I should have known this from my BSEE courses 30+ years ago! Thanks Keith.

My post was based on observations. An oscillascope is very helpfull in diagnosing and understanding ignition system operation. When viewing the secondary ignition it can be determined that introducing an air gap will cause the initial voltage spike to increase.

I’m old enough to have worked with the old Sun Distributor Tester in conjunction with a Sun Scope. With that setup it is possible to see the operation of the primary and secondary system and the result at the plug(albeit dry and at 1 Bar). I won’t argue ohms law, though.

“But it must also make the spark cooler, no?”

My first instinct is to say no, it is actually hotter, but you define exactly what you mean by cooler. Per unit time, i.e. per microsecond, the intensity may be slightly less, but with the overall duration, the total energy of the spark is greater.

Think of impedance matching. If the load is too small, you get high current but low voltage. If the load is too high, then you get a high voltage but little current. Total power in both cases is low. But at the perfect impedance match, you get half voltage and half current, but the maximum power transfer.

In this case, we are trying to get the maximum transfer of current for the longest duration possible.