Ignition, general question


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CBX1000chris
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Ignition, general question

Post by CBX1000chris »

Hello everyone, and as it's my first post in 2021, Happy New Year.

I'm working on a bike, not my CBX, it's actually a Honda V4 and I have a question if anyone can help please. The spark seems very weak, if I take a plug from this bike and pop it in my FTR223 I get a great big fat spark so I know the plugs are OK. I was pondering the ignition wiring on the bike and to prove to myself I thought knew what I was doing I gabbed a spare coil from a box of bits to see if I could bench test a Honda coil.
The spare coil is off a VF750F and is the typical two low tension spade terminals and two HT leads. This coil tests OK to the Honda specification in the maintenance manual, 2.8ohms primary between the spade terminals, 22kohms between the plug caps
I wired the test rig up on the bench as follows:

First spade terminal to a battery positive
Second spade terminal to a flying lead
Plugs fitted to the HT caps and then the plug threads joined together to simulate being screwed into a head, (the current flow on this system is through the first HT lead to the plug, plug to head, head to plug, second HT lead back to coil).

When I placed the flying lead on the battery negative and then removed it I expected to see both plugs spark as the magnetic field induced in the primary winding collapsed but nothing. Reversing the polarity on the primary didn't help. If it was an old bike with contact breakers the test I'm doing would seem representative, I can't see why it would be different with the coils from an electronic ignition system, when all's said and done isn't a coil just a transformer?

So my knowledge and understanding is lacking. Maybe just dabbing a wire against a battery terminal is flawed?

Forgetting the troublesome bike for the moment, can anyone explain why the bench test didn't work as I expected it to?

Thank you in advance,
Chris.

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shiskowd
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Re: Ignition, general question

Post by shiskowd »

You need to ground the plugs that are together to the negative on the battery using a heavier lead. Should work unless you have a broken HT wire or a bad coil. Also, if you're using old resistor caps, the resistors could be open.
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Long Sleeping '79 CBX Restoration > viewtopic.php?f=102&t=11699

CBX1000chris
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Re: Ignition, general question

Post by CBX1000chris »

Hi shiscowd,
Thank you for your reply.
I did try earthing the threads of the plugs to the battery with a jump lead but no joy. I figured that the HT leads and resistors would be OK given I got a reading of 22kohms between the plug caps and this was within specification, are you saying that continuity might be OK when testing with a meter but breakdown under HT voltage?
I think the current flow is as shown in the red line so not sure why earthing the plug threads to the battery -ve would make a difference but I am lost so not sure. So long as the threads are connected together wouldn't it complete the circuit?
VF750F Ignition.jpg
Regards,
Chris
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shiskowd
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Re: Ignition, general question

Post by shiskowd »

Curiosity got the best of me and I tried the same with a an old dual output coil, fresh plugs, new resistor caps and got the same result as you (no spark at the plugs). My coils are old but they check out, I get 4A of current flowing in the primary but no spark on the plugs when the coil discharges. I've done the same test with single output coils and worked as expected.

I found a posting by Mike Nixon that describes the same test as we're doing with the spark plug bodies connected to the negative terminal of the battery. It should work every time as Mike suggests... :? I must have a bad wire or coil in my test setup. I'll give it another go when I get my new coils.

https://www.cbxclub.com/forum/viewtopic ... 515#p23515
2022 KTM 890R, 1982 Pro-Link, 1979 CBX 'Z, 1975 Moto Guzzi 850T
Long Sleeping '79 CBX Restoration > viewtopic.php?f=102&t=11699

CBX1000chris
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Re: Ignition, general question

Post by CBX1000chris »

This has evolved into a personal challenge between me and the bench test, I've forgotten all about the problem with the bike!
Known good coils are coming off another bike today, if I can find the time, and I'll repeat the test.

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herdygerdy
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Re: Ignition, general question

Post by herdygerdy »

Can't comment on the bench test CBX1000chris, but perhaps you are getting HUGE voltage drop at the coils (when compared to the + & - of the battery terminals.

The highly talented and respected Mr Mike Nixon told me about this test that may help to solve your issue.

Set your multimeter to 12v. Ignition on.

Put the + lead from the meter onto the battery positive terminal.

Then put the negative lead from the meter onto the 12v input terminal on the back of the coil.

This test measures the voltage (ie; electrical pressure) differential between the battery terminal and the voltage actually arriving at the coils.

If you are getting a reading, then this suggests you are getting a voltage drop as the circuit traverses many old wires and corroded terminals and connections on its very long path from the battery to the back of the coil.

On my CBX I had nearly 3 volts of drop. Rather than pulling my 40 yr old wiring loom apart, I solved it quickly by installing an ignition coil relay. This uses the output wires from the kill switch as the input trigger on a good quality relay, to feed 12v from battery directly to the back of the coils. Immediately had better starting (especially from cold), smoother running and a visibly fatter spark. YMMV.

CBX1000chris
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Re: Ignition, general question

Post by CBX1000chris »

Hello all,
Well this was eating away at me and I decided I needed to increase my knowledge of CDI ignition systems. I 'think' I know why I'm not getting a good spark in the bench test, basically my procedure is flawed.

In a CDI system a capacitor is allowed to charge either from a generator (if no battery is fitted) or from the battery if one is fitted, this capacitor stores a lot of energy. The ignition pickup triggers a thyristor that allows the capacitor to discharge through the ignition coil and this discharge happens very, very quickly because, unlike a battery, a capacitor has a very low internal resistance. The energy stored in the capacitor is discharged very quickly into the low resistance of the ignition coil primary winding and this induces a voltage in the secondary winding and the plug sparks. There is a great animation on YouTube.

https://www.youtube.com/watch?app=desktop&v=0yK3Opq_i0M

So it's not the same as an old points system where the points opening causes the primary winding magnetic field to collapse inducing a voltage in the secondary winding creating a spark. I now think with a CDI ignition it's the speed of the voltage rising in the primary winding that creates a spark. My test was never going to work.

I'm still very much in the 'I think' phase and would welcome comment. Happy to be shot down in flames.......

Regards,
Chris.

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