SPARK TESTER
by Jan Ridders

THERE ARE several possible causes when an I/C engine runs irregularly and/or stops suddenly. One of the causes is irregular dropout of the spark on the spark plug. It is very useful first to determine if this is the case before going on to other and, maybe, needless investigations.

Of course it is easy to find out if there is a spark by pressing the points (electrical switch) for the primary current of the high tension coil and check for a spark while holding the high tension cable near the mass of the engine. But that doesn't always mean that nothing is wrong with the ignition system. It can happen that the sparks do not occur at every engine cycle when the engine is running, and that is less easy to find out.

In that case the engine will run irregularly and most of the time will finally stop. There are several possible causes of this phenomenon:

- Too high contact resistance of burned-in or contaminated points causing an insufficient primary current through the high tension coil. Mind that the electrical resistance of the primary coil is also only a few ohms while the 3 to 5 amps current is rather high.

- Floating points of the switch at certain speeds. In general points that are made for industrial engines will not suffer from this, but any other arbitrary electrical switch can. Most of the time they are not made for high mechanical switch frequencies.

- Rooted or wet spark plug. This is a well known phenomenon with misadjusted classic carburetors. By the way when using my ‘Petrol Vapour Carburetor’ this does not happen.

- Spark flash-over at other places in the electrical circuit than on the spark plug due to insufficient isolation of parts with high electrical tension.

Maybe measuring the current through the primary coil can give any exclusion in the first two cases, but for sure not in the last two cases because this current will be normal then.

To check for a regular spark when the engine is running I made a childishly simple little tester; see above drawing and photo A. Two brass props with spring steel wires soldered in it are glued in a small glass tube with the well known acrylate instant glue. The distance between the somewhat pointed ends of the wires is about 0,5mm. When this tester is connected to the spark plug and the high tension cable (see photo B) there will be a tiny but easily visible spark between the wire ends simultaneously with the invisible spark in the spark plug in the cylinder. If you see no or an irregular spark in the glass tube you can be sure that this is also the case for the spark on the spark plug, as the two sparks are connected in series.

Apparently, the little spark in the glass tube has no or negligible influence on the spark on the spark plug; all my engines run with or without this tester. Of course it is not the intention to keep this tester connected when everything is found to be OK.

Only the 0.5mm dimension between the pointed wire ends is of any importance, first to make the spark flash-over easy to occur (we don't want to test the tester) and secondly to keep the spark as small as possible to avoid any significant energy loss. All other dimensions are not critical at all so you can deviate from them as you wish.

Because the tester is air-tight the small amount of oxygen in it will disappear rapidly after some sparks, so the wire points will not oxidize any further.

So this tester is especially useful in the case of suspect irregular sparking which is rather difficult to discover on a running engine. I really think there must be something like this existing already, but I never saw it till now. Of course it is possible to use an electronic oscilloscope but that is not available to many and you need some special knowledge to work with it. Besides it is a little bit risky to connect such a scope to the high tension part of this ignition circuit.

Discussions on internet forums about my spark tester triggered me to pick up a small discharge lamp that is, or rather was, used for signal functions in non electronic devices like flat-irons. I worked for 40 years at Philips Lighting and these kind of lamps were made there; see photo C. These are discharge lamps with two cylindrical electrodes with an emitter on it, and the lamp is filled with low pressure neon. The lamp ignites at every voltage higher than 220 volt and the current through it is normally

limited to about 0.5 milli amps with a 50 KOhm resistor.

Connected between the HT cable and the spark plug (see photo D) this lamp will light at the high tension pulses of the coil and the current through it will be normal for this lamp since the high tension current of the coil is also anywhere between 0.5 and 1 milli amps; so the 50 KOhm resistor can be omitted here. The reddish discharge reacts very fast but you can follow every spark on the spark plug at least with engine speeds up to about 1000 RPM.

You can see that on the video where this lamp is connected to the spark plug of my Disk Valve 4-stroke Engine running at about 800 RPM. The somewhat irregular discharges on that video are a result of stroboscopic video effects; in reality the discharges were very regular in this case of the nice running engine.

The engine runs as well with or without this spark tester so I may conclude that it has no or negligible influence on the energy of the spark on the spark plug.

Obtaining this type of signal lamp may be a problem nowadays, but there must be still a lot of them in this world, although it is possible that you have to dismantle an old device for one.

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