mag not working - was fine!

[Rob Harknett]

What problem??? its all my bikes have needed after a lay up, before which they were fine.
What mag are you got? If SR1, clean the spring and contact point on the coil, pull out the spring a bit so it makes good contact. If not SR1. remove the plug cable to mag connector, Check the fitting is not cracked. Check the wire is clean and the little disc it goes through, they can oxidise. Check the brush is OK and moves freely, that can stick. It is usually just these little general maintenance jobs that need doing. If it was OK and nothing else has been done that could have caused damage. Quite often some may fear the worst, replace a part, and its OK. All they may have done is just cured a doggy connection.

[Chris Berry]

Correct me if I'm wrong but continuity checks don't achieve to much as the condenser still provide a circuit. What you need to check is resistance. On the Australian Historical Motorcycle Club web page there are plans to make a timing device to compensate for the condenser. Alternatively you would need to disconnect the condenser from the circuit. Below is an extract from our Historical Motorcycle Club website

MAGNETO TIMING TOOL Nov 2016 Your magneto produces a spark at the instant that the points open. In order to adjust the
ignition timing you need some method to indicate exactly when the points open.
Electrically, the difference in resistance between points open and points closed is equal to the resistance of the magneto primary winding – around 0.5 ohms – almost impossible to indicate with a multimeter.
Traditionally you either test the points with a thin feeler gauge or you pull a wire off (or a screw out of) the points inside the magneto so that you are metering the points in isolation.

[Groily]

There will be continuity with the points open as Chris says.
What you will see with them open is the 0.5ohm or so of the primary winding of the coil, because the points are in parallel with it as they obviously have to be (and with the condenser too, but that is quite irrelevant to this test). You'll only see an open line with the primary disconnected as Chris also says.
With them shut, the reading would be 0 ohms if your meter were sensitive enough to show the difference.
It's why you can't set things using battery & bulb etc the way you can with a battery/coil system - the buzzers available measure the very small difference in resistance between points open and shut.

Lay-ups often cause oxidisation as mentioned. Often you can see nothing, but getting the points squeaky clean restores the spark.

[clive]

Short story are you sitting comfortably?
I laid up my Model 18 combination because a broken and unmended collar bone made it painful to ride it more than 30 miles. It stood unused taking up space in the garage for 6 years. I decided to sell it and pulled it out but it would not start. Drained the sump it still would not start. Then another garage came up for rent in the rather expensive Secure site I use in London. Ah! I will rent an extra garage for 3 months and get the bike going so I can get the full value for it ( by now it was pretty tatty) as a runner.
Then a G11CS came up for sale in pieces and with the new space in the original garage was easily worked on. 2 years later I finally got round to working on the combination. NO spark, that is too simple! Cleaned the points and it started first time. Sold it as a runner. Trouble was what I got for it only just covered that two years of extra garage rental.
Moral of the story always try cleaning the points if a running bike will not start after a period laid up.

[Rob Harknett]

Its worked for me over the last 58 years Clive, drain sump, bit of cleaning, take the bottom off the carb, turn the petrol on, let it flood and wash itself. The only thing I have ever worried about has been starting after doing just that. Often just moving the kick starter lever a bit, when its about to fire, that's just what happens. A mighty unexpected kick back. I have only ever had to replace burnt out points and cracked Bakelite /plastic lead connectors. On SR1 mags, points and gasket. I have only ever replaced one condenser, just for the hell of it. The new one did not work, so put the old one back on. All my bikes have always been good starters. I have never bothered testing a mag. There's something to be said about, If it aint broke don't fix it.

[Mick D]

Hi

I had a very similar issue on my G3 coil ignition - maybe the cause will be applicable to mag ignition.

The CB set had an insulating collar and washer that fitted over the stud retaining the CB leaf spring, the leaf spring had chaffed through the washer and was grounding to earth all of the time - replaced washer and all fine

CB Fault.png

Regards Mick

[Rob Harknett]

Important little washer Mick, have to be careful not to forget it or put it in the wrong place. Also arrange wires tidy. I once changed points on Reliant. Did nearly a 1,000 miles through Europe to the far north of Denmark. The engine stopped. I found the rotor had worn through insolation on a wire. Used a plaster from my First Aid box to insolate it. Gave me a fright, so far from home. Lucky I went straight to the distributor and spotted the damage. I make sure wires are nicely routed, and stay where I put them now. It is these little lessons you learn along the way, pass on, and hopefully help some one else spot the problem, when / if it happens to them.

[StephenG80]

Thats my thinking Mick - I don't get that people think its not continuity causing the problem when the points are open! - if that was the norm then the points would be pointless!! - I did all the school boy stuff like cleaning points etc etc before posting. I think its the insulator under the top points tower and/or the retaining bolt that have gone awol. Time will tell.

[Groily]

You can't draw straight comparisons between coil ignition and a mag Stephen, they simply don't work quite the same way in terms of continuity at the contact breaker for reasons explained already.

With a battery and coil the cb acts as a simple switch to make and break the supply from the battery to the independent coil it supplies directly.
With a mag, there is no external power source, just a small generator - the primary winding of the magneto.
When that turns in the magnetic field created by the permanent magnet in the mag body and the points are closed, it creates a small alternating current, which the points' opening interrupts.
Interruption induces a massively higher voltage in the secondary or HT winding.
How high depends on the number of turns of the secondary as compared to the primary - usually about 40 x more fine wire than the primary's heavier gauge.
The condenser acts to prevent/reduce sparks arcing at the points as this happens, and thus the majority of the energy of the small primary voltage created by the rotating primary winding is transformed into a massively higher voltage in the secondary winding. Which lets it out to the spark plug via the slipring and HT pick-up.

When the points are open on a magneto, there will always be the resistance - very small - of the primary winding across them as long as the contact breaker is attached to the coil's primary winding by its centre screw (N1) or by the live low tension wire direct from the coil to the points (SR1). The other end of the low tension winding is permanently earthed along with the HT winding - and its simple resistance is around half an ohm in both cases as already mentioned.

So whether your magneto is the N1 you seem to be describing, or an SR1, the situation remains the same. The N1 has a rotating coil and a static magnet whereas the SR has a rotating magnet and static coil, but the result is the same.

The points very definitely aren't pointless - even though they don't do what you seem to expect. No continuity is equally definitely not what you want - continuity is essential. If there were an open reading across the points with them open and with the mag fully assembled, the magneto wouldn't work at all!

[MikeM.]

Connect plug to lead, hold plug in hand and place other hand on bare metal part of handlebars. give kickstart a good swing and if mag isn't working you will still have a hold of both handlebars and plug.