DIY! How to Diagnose Ignition System Using Ignition Spark Tester?

1016motomeye.jpg How to

One of the things to check when the engine is not working well is the ignition system. It's easy to assume that if you remove the spark plugs and sparks fly, the ignition system is fine, but the load on the plugs is different in the atmosphere than in the combustion chamber. In such cases, an ignition spark tester that allows you to adjust the spark gap at will will help you determine if the cause is the plug or the ignition coil.

Remove the spark plug and you'll know if the ignition spark flies or not.

101602-5.jpg No matter what kind of high performance engine you have, if the spark plug doesn't send sparks flying, the mixture won't ignite. Many recent models of motorcycles require the removal of many parts, such as the cowl and gasoline tank, in order to install and remove the spark plugs. However, we want to inspect and replace these parts according to the mileage so that we don't have to deal with engine malfunctions caused by parts costing a few hundred yen per plug.

A good engine in good shape requires three main elements: good mixture, good compression, and good spark, and if the engine won't start, or starts but doesn't feel good, these three elements should be checked. The easiest of these elements to check is a good spark, or spark plug.

Even though it's easy, it can be a hassle, as many modern motorcycle models require you to remove the cowl and gas tank before you can get to the spark plugs.

But even if you try to check the mixture, it's not easy to know if the gasoline and air are mixing properly and if the gasoline supply is not excessive or inadequate. A clogged air cleaner element or a dislodged or cracked air intake system pipe can cast suspicion there, but if it's a carbureted car, it's not easy to check the carb settings or the fuel injection program of an injected car.

The same is true for compression, if you remove the plug and push the cell motor, air will swoosh out of the plug hole, but without a compression gauge, there is no way to measure the pressure and know if the cylinder or valve is in good shape.

So even if you have a hard time getting to the plug, removing the plug from the cylinder head, removing the plug cap, and turning the cell motor with it pressed against the engine, or even pressing the kick pedal to check for sparks on the electrodes to see if you can see and check for sparks in the electrodes, is the easiest of the three factors This is a checkable item.

If the spark doesn't fly, the cause can be divided into two types of problems: one is in the plug itself, such as a short circuit because the plug is dirty with carbon or wet with excessively dense gasoline (too dense gasoline can be a problem with the mixture), and the other is in the ignition system, where no high voltage is generated from the ignition coil. It is available.

By the way, when checking for spark plugs for sparks, it is important to always press the cell button with the plug's screw or keyed bent outer electrode in contact with the metal part of the cylinder head and grounded. If you hold the plug in your hand and turn the cell motor without grounding it to the engine, or if you press the kick pedal, 20-30,000 volts of voltage will flow through your body and electrocute you.

You can tell if sparks fly on the plug or not by placing the removed plug against the engine and turning the cell motor or stepping on the kick pedal. If you hold the plug cap and the threaded part of the plug is touching the engine, you should be fine, but if you hold the threaded part of the plug and turn the cell while it's floating off the engine, you'll get an electric shock, so be careful.

The voltage is high but the current is low, so an electric shock is not fatal, but in some cases you may feel a big shock from your fingertips to your shoulder.

If you can hear sparks crackling from the tip of the plug, you can know that ignition is taking place, just in case. I say "just in case" because whether or not it is the energy required at the right time is another matter.

In the case of contact breaker point ignition, or so-called point ignition systems, where the ignition is mechanically controlled, the ignition timing must be adjusted by the user. A "good spark" needs to fly just before the piston reaches its upper compression dead center, but depending on the adjustment of the point, it can also fly in the wrong place. Thus, there is the possibility that the sparks are flying on their own, but when they are assembled into the engine, they are not in good shape and cannot be started.

As for the second ignition energy, the voltage generated by the ignition coil may be reduced, causing the spark to become weaker. As the coil itself deteriorates over time, cracks in the housing and loss of insulation can cause the plug to ignite as the temperature of the coil rises, or electricity can leak from the connections between the coil and the plug cord or plug cord and plug cap, weakening the ignition.

Therefore, you may not be able to get to the cause of the malfunction if you determine that there is no problem with the ignition system, just the one point that sparks fly on the plug you removed.


  • Point 1 - Putting the spark plug against the engine and turning the engine will tell you if the sparks fly or not.
  • Point 2 - Be careful not to touch the metal part of the plug to the metal part of the engine, otherwise you will get an electric shock.

Spark load is different under atmospheric pressure and under high pressure compression in a combustion chamber.

This is the ignition spark tester. Some products have the same shape and different names, but there is no difference in function.

What if I've removed the spark plug to check for a "good spark" and it's not enough to make the spark fly?

If the ignition energy is reduced, the spark plug may be weak or even lose fire when installed in the engine, even though the sparks fly in the atmosphere. The reason for this is that in a combustion chamber filled with a high-pressure, compressed mixture, there is a force in the combustion chamber that also compresses and tries to crush the electric spark's seed fire.

If you measure the pressure in the combustion chamber with a compression gauge, which can measure compression pressure, you'll find about 800-1300 kPa, depending on the engine. Atmospheric pressure is about 100 kPa, so the pressure is compressed about 10 times more. When the mixture explodes, the pressure in the combustion chamber can reach up to 5000 kPa.

Because of this, even if sparks fly in atmospheric pressure, they may not ignite properly in the combustion chamber and lead to engine malfunction. As a symptomatic treatment for the weak ignition energy and difficulty in getting sparks to fly in the combustion chamber, this is also a roadside repair technique for older or out-of-print cars, but temporarily narrowing the plug gap may allow the sparks to fly under the pressure.

However, when the gap is small, the spark fire is also small, so the ignition-burning performance will be less than when a large spark is flying in the proper gap. Nevertheless, the small energy of the ignition system is better than no sparks flying at the proper gap, so this kind of action is effective.

The gap between the outer and central electrodes of a spark plug is generally 0.7 to 0.It is around 9mm. In the SPII kit made by AS Uotani, which is known as a high performance ignition unit, it is 1.1 to 1.A gap as wide as 3mm is specified because the SPII's control unit and high-powered coils can produce a higher voltage than the stock ignition system.

If you can't diagnose the ignition system by just "crackling" in the air and sending sparks flying, what can you do? This is where an ignition spark tester comes into play. This tool is sometimes referred to as a spark plug tester or a plug gap tester.

Instead of being a "tester," it has a simple construction with a terminal that plugs into the plug cap and a threaded, clip-on cord with a pointed tip that allows you to adjust the gap, which may make some people wonder how much functionality it has from its analog look.

However, there is a secret in this simple mechanism that allows us to diagnose the capabilities of the ignition system.

Earlier, I wrote about the example of sparks flying in the atmosphere but being crushed by pressure in the combustion chamber.

Just as it's harder to ignite under compression pressure than under atmospheric pressure, sparks are harder to fly when the plug gap is wider than it is narrow, even under atmospheric pressure. So the ignition spark tester tries to measure the performance of the ignition coil by adjusting the plug gap as desired.

The tester we'll be discussing here has a number of numbers written on the body of the tester, and depending on how far the sparks fly as you widen the gap, you'll be able to see the voltage generated by the ignition coil. A typical ignition coil generates 20,000 to 30,000 volts, so if you widen the gap to 20 on the tester and sparks fly, you can judge the coil's performance to be normal. 30 indicates 30,000 volts, 40 indicates 40,000 volts.

This would be a more reliable result than judging only the 0.7-0.9mm gap in the spark plug, which would be measured under high load.

Plug the plug cap into the terminal area, ground the leads to the body of the car, press the cell button, and the voltage generated by the ignition coil will cause an airborne discharge. The blue-white sparks and the jizzy ...... sound of the discharge are slightly terrifying.

Turning the adjustment screw in the loosening direction widens the gap and increases the required voltage for discharge. If one of the two coils is 30,000 volts and the other is less than 20,000 volts, the lower voltage will be required. There is likely to be some kind of problem or glitch.

  • Point 1. Sparks may fly under atmospheric pressure, but not in a compressed mixture in a combustion chamber.
  • Point 2: The ignition spark tester can measure the voltage generated by the ignition coil.

Inexpensive but more useful ignition spark tester than you can imagine

In the case of a two-cylinder engine with one cylinder and one coil, one of the coils may be malfunctioning, which can cause idling problems and plug burn. Reviewing the carb settings and measuring the compression pressure as well as measuring the voltage generated by the ignition coil will help with troubleshooting.

In the case of a 4-cylinder, 2-coil system with simultaneous fires for Nos. 1 and 4, and 2 and 3, when the plugs go out of fire, 2 cylinders will be set for each. Therefore, if the #1 and #2 plugs are in conflict, the cause is probably the carb setting rather than the ignition system. The reason the AS Votani SPII high power coil can generate 40,000 volts compared to the stock coil's 20,000 to 30,000 volts is because of the combination of coil and control unit.

Ignition spark testers, sometimes called spark plug checkers, are sometimes interpreted as devices that check to see if sparks are flying in the plugs. That certainly captures one aspect of this tool, but the essence of the tool is that it can tell you the voltage generated by the ignition coil by adjusting the gap.

This capability is especially useful with four-cylinder engines. Typically, four-cylinder engines have two ignition coils, with one coil igniting the plug in the first and fourth cylinders and the other coil igniting the plug in the second and third cylinders. So if one of the ignition coils fails, either 1 or 4 or 2 or 3 will have a poor ignition condition.

If this happens, the first step is to swap the two coils and see if the symptoms show a change. If the faulty plug is reversed here, you can diagnose that the coil is the cause of the problem. If changing the coils does not change the condition, then there may be a problem with the plug, or if you are experiencing fire loss or cover symptoms, then there may be a problem with the air mixture.

Next, measure the voltage generated by the coils with an ignition spark tester, if sparks fly at 20,000 volts, you're good to go, but if you have two coils with four cylinders, you can measure the voltage generated by each to get a sense of the relative performance difference. I don't know if it can be done, but 10,000 volts and 30,000 volts make a reasonable difference.

Particularly in the case of point ignition, a small point gap lowers the primary voltage flowing through the coil and proportionally lowers the voltage generated by the ignition coil, making it more likely that a voltage difference between the two four-cylinder coils will occur. In this case, it comes from the contact breaker gap adjustment rather than coil performance, and an ignition spark tester will tell you that you have done so.

Some of the spark plug checkers are designed to light up when you insert them between the plug cap and the plug, or by placing them along the plug cord. These can check whether the plug to be tested is ignited or not, but they cannot tell you the voltage generated.

The simple construction ignition spark tester is an inexpensive tool that can be purchased for around 10 USD, but it is one of the most useful testers that can easily check high voltages in the tens of thousands of volts, which are generally difficult to measure.


  • Point 1 - The ignition system can be diagnosed by comparing the generated voltage of multiple ignition coils relative to each other.
  • Point 2. Although simple and inexpensive as a measuring device, you can get important information on engine maintenance.