Poor spark on the 40 plus

[raysmith]

I'm having difficulty getting a decent spark from my 40 plus. I've cleaned the points and set them to 20 thou, replaced the spark plug and HT lead, and done the 5 minute spin thing. Still I can get it to start. I am slightly confused with a continuity check. If I put one lead of my multi tester on the coil at the point where the LT lead is soldered on, no matter where I put the other end, on the points or anywhere on the body I have continuity. My limited knowledge of engine electrics suggests this can't be right??

Ray

[charlesp]

Three questions:

1) Have you got a spark and if so what colour is it?

2) What reading are you getting between the HT contact and earth?

3) Which version of the ignition is this - Villiers or Wipac? (serial number may help here)

If you have actually got a spark then we're half way there!

[Dr.Lighthouse]

As you might know, I'm new into Seagulling, but I do have 30-odd years of 50 - 250cc motorbike engines experience.

If your multimeter is an analogue one (with needle) you'd have to short the leads and calibrate to "zero" (end of scale).
With digital meters short the leads as well, and take notice of the reading: it will be low, but hardly ever 'zero', due to the resistance of the leads.

Now measure the resistance between the primary (the small wire going to the points) wire and ground.
It should be very low. Like 0.5 to 1.5 Ohms.
(that's why you have to calibrate the meter)

Next measure the secondary (high-tension) wire and ground.
This should be quite high, like 6000 to 13000 ohms.
(you probably will need to switch the meter onto another scale)

Sometimes, a coil will work OK when cool but fail when it warms up.

Good luck . . .

[Swami]

I've had a similar problem recently. In the end the cause was found only after completely stripping the sparking system to find that the old insulation on the primary coil wire (the one going between points and coil) was cracked and worn through in several places causing intermittently good and bad spark plus hard starting. If the other simpler ideas above don't work, you may find the cause UNDER/NEAR/AROUND the points box. Hope this helps.

[raysmith]

Three questions:

1) Have you got a spark and if so what colour is it?

Blue at the very rare time there is one.

2) What reading are you getting between the HT contact and earth?

The multimeter doesn't move no matter what scale it's set at.

3) Which version of the ignition is this - Villiers or Wipac? (serial number may help here)

Villiers

[raysmith]

As you might know, I'm new into Seagulling, but I do have 30-odd years of 50 - 250cc motorbike engines experience.

If your multimeter is an analogue one (with needle) you'd have to short the leads and calibrate to "zero" (end of scale).
With digital meters short the leads as well, and take notice of the reading: it will be low, but hardly ever 'zero', due to the resistance of the leads.

Now measure the resistance between the primary (the small wire going to the points) wire and ground.
It should be very low. Like 0.5 to 1.5 Ohms.
(that's why you have to calibrate the meter)

Next measure the secondary (high-tension) wire and ground.
This should be quite high, like 6000 to 13000 ohms.
(you probably will need to switch the meter onto another scale)

Sometimes, a coil will work OK when cool but fail when it warms up.
Good luck . . .

The resistance between the primary and ground appears to 0.009 ohms and I'm not getting a reading on the secondary.

Thanks for that

[Dr.Lighthouse]

The resistance between the primary and ground appears to 0.009 ohms and I'm not getting a reading on the secondary.

With a reading like that it seems that you are using a digital ohmmeter.
To get a correct readout, you will need to switch the scale to the lowest possible setting for the primairy coil.
On my (cheap) multimeter this is 200 Ohms.
If you try measuring at a higher setting, the readout will definitely be incorrect.
A reading of 9 thousands of an Ohm is quite remarkable:
a piece of copperwire with a lenght of 60 cm and a thickness of 1mm² will have a resistance of exactly 0.009 Ohm . . .
(the probes of your meter?)

Assumed that the readout is correct, this can have two causes:

1°) your points are closed
2°) there is a short circuit in the primary system

Your post triggered my curiosity, so I decided to measure my Century's ignition system to offer you an example.

The measured resistance in the primary coil was 0.9 Ohms
(measured over open points)
Please notice that this is exactly 100 x your reading, so it might have to do with misreading / wrong scale !

For the secondary coil, I removed the HT-lead and metered between "ground" (i.e. the metal of the flywheel) and the HT-out socket that is to be found in the 'blackbox' under the flywheel.
I got a reading of 5000 Ohms, with the meter set to "20k".
(the meters readout was '5.0' but because the scale was 'k' this means 5K Ohms, or 5000 Ohms)

With the meter still on the 200 Ohm setting, the readout was '1' meaning an infinite resistance.

So it is important to use the correct scale, or the readout will be worthless.


Remember Ohms Law: R = U / I

whereby
R = resistance in Ohms
U = potential in Volts
I = current in Amperes

If you want to make sure your reading is correct,
take a 60 Watt - 12 V headlamp bulb.

These bulbs have a resistance of about 2.4 Ohms

Ideal to check if your meter settings are OK.