Testing ignition coils

[Craizeehair]

I have had trouble with the left side not firing a couple of times now, I have carried out some tests that I have found and hoped I could get any informed opinions before I start spending more money on stuff I don’t need.

I have carried out tests on my ignition components as per the Common Motor videos with a test light and I have tested resistance at my coils with my multimeter.

The right side that is sparking had 6.9 ohms across the two wires from the coil and 14k ohms from the plug cap back to the coil.

The left side has 4.3 ohms at the two wires and 14k ohms from the plug cap back to the coil.

Could the left coil be bad? From what I’ve read online, if it has between 3-5ohms it should be ok, any opinions?

There is a disparity between the two, almost a 3 ohm difference but could that be the cause of no spark?

The bike was running on both cylinders until I decided to take the tank off and investigate the hanging idle and erratic tick over??

[Bryanj]

4 to 7 is a 50% difference and makes me wonder if one has been changed before

[Craizeehair]

4 to 7 is a 50% difference and makes me wonder if one has been changed before

I did wonder myself, have I missed anything?

I’ve put new plug caps on, cut back the leads for a good connection with them, tried swapping the plugs and tried a new plug but still no spark. Tried bypassing the points and striking a lead on the engine casing to get a spark at the plug but still nothing.

Next logical move is coils… or should I be trying something else?

[AshimotoK0]

7 Ohms sounds high .. No resistance figures given in any of my manuals only 'Service Tester' info. I know that Honda 4 cyl.  coils are typically 4.5 Ohm primaries but I will check a Honda twins 12v coil to see whata typical one measures and get back to you. Usually coils reduce in resistance when they are faulty .... if they overheat then turns can get shorted and hence you get reduced resistance across the primary coil.

[Craizeehair]

I thought 7 was high based on what I’ve seen online and read, 3-5 seems the norm but that’s the one for me that isn’t sparking and the 7 ohm is.

Does anything else come into play here or have I checked everything I need to and could it come and go if it was on the way out, as it has worked and not worked?

[Bryanj]

Try measuring the voltage black/white to earth then the other lead to earth , both when disconnected from points, compare the coils.
Yes you should get a spark from end of ht lead to earth when you earth the points lead then disconnect it, plus a small spark from end you are grounging, more difficult to descibe than do

[AshimotoK0]

Have you checked or replaced the condensers? If they are modern Daiichi's ...they often fail pretty quick as many fakes around made in China

[Craizeehair]

No, I haven’t checked the condensers, apologies for silly question but will they affect spark?

I did remove the wires from the points and manually spark it to ground as I think Bryan has suggested, I still had no spark then.

[AshimotoK0]

No, I haven’t checked the condensers, apologies for silly question but will they affect spark?

I did remove the wires from the points and manually spark it to ground as I think Bryan has suggested, I still had no spark then.

Condenser is essential for a decent spark.

For us Anoraks out there: 

The capacitor (called a condenser up to about 75 years ago) forms a resonant circuit with the inductance of the coil. While the points are closed, current (and the associated magnetic field) builds up in the coil. When the points open, the current is diverted to the capacitor due to the collapse of the magnetic field. The result is a sinusoïdal rise in voltage across the capacitor, which voltage is also across the primary winding of the ignition coil. This produces a large voltage across the secondary (which is connected to the spark plug) igniting the fuel mixture in the cylinder.

The width in time of the voltage pulse is determined by the inductance of the primary of the coil and the capacitance of the “condenser”. The values are chosen for optimal ignition.

If you remove the capacitor, the voltage will try to spike higher but will be limited by arcing at the contacts. But in addition the width of the resulting voltage pulse at the spark plug will be much much shorter and the spark energy much lower. This can result in inefficient ignition (or none at all). Of course, the contact arcing is also undesirable but that’s not the primary reason for the capacitor.

[Craizeehair]

Might take me a while to digest that!

Would one side still spark and one not if the condenser was no good?

[Laverda Dave]

That explanation has taken me back to when I was 15 and sitting in a physics class with my tutor, Mr Tombs explaining and demonstrating Ohms law and magnetic flux etc. A few weeks later I witnessed an impromptu demonstration of how capacitors worked  in our motor engineering class when we were servicing the metalwork teacher's Singer Gazelle car. One of my fellow students was dared to stick his finger in a plug cap while the engine was cranked over by another student. He didn't keep it in there for long!

[AshimotoK0]

Might take me a while to digest that!

Would one side still spark and one not if the condenser was no good?

There are condensers for each side but two in a common package on the Honda twins

[Craizeehair]

I got the bike firing on the left today with a new coil from DSS, popped it on and a good healthy spark straight away.

[taysidedragon]

I got the bike firing on the left today with a new coil from DSS, popped it on and a good healthy spark straight away.

Good news. It's a relief when you solve a problem like that. 👍