I was out on a test run of around 5 miles on the B33 last week checking for leaks after rebushing the outer gearbox case. Less than 1/4 mile from home the engine died and wouldn't start. Luckily I could coast home downhill. There is no spark at the plug or HT lead. I have checked and cleaned the magneto brushes, but still no spark. Before I get the magneto onto the bench, am I missing something obvious? A previous owner spent a bit of money on a full service including an exchange armature in 2014 and the bike's done little mileage since. Strangely, the armature was replaced again after a month as the replaced one failed!
Any thoughts chaps?
fuse?
connector?
earth?
thats where I would start before stripping the mag.
Agreed, always start with the obvious.
There's no fuse or connectors in the magneto circuit. When I got the bike nearly two years ago, I chatted to the magneto guys re the earth terminal which had no lead fitted. They felt that it shouldn't be important as it should earth through the frame, but when I rewired the bike, I added an earth. Up until last week it always started and ran well.
Hi,
I found this place a good resource, link here:
http://brightsparkmagnetos.com/DIY/index.htm
Best wishes...
An obvious thing. Is the magneto armature rotating. Make sure that the points spring is not touching the cam on rotation,if the little screw backs off the spring will move on its slot. Check that the commutator is not contaminated with carbon, there is some soft brushes around. Is the little brush still in the black pickup.
This'll keep me busy for a while. I'll report back.
Cheers
Also check the points are actually opening and the HT pick up hasn't worked loose (there is no commutator, that's in the dyno part)
Could also be a duff plug, try getting a spark direct from the HT to earth, or a different plug.
good luck
Must put brain in gear. Confusing mag with dynamo.
The mag part has a slip ring which can become contaminated with carbon from the HT pickup brush.
Ok so here are my preliminary findings:
I first of all looked at the HT circuit from slip ring to plug cap and everything looked good
My next test was to check the circuit across the points - there was continuity whether the points were open or closed
Resistance test between armature and body (earth brush removed) - gave a reading of only about 1.5 ohms
My thoughts are that with little resistance across the magneto, the bearing insulator is no longer effective, and a strip down is needed to explore things further.
Happy days
Hi,
Priory magnetos would be my go to, link here:
https://sites.google.com/site/priorymagnetos/services
Best wishes...
Quote from: Greenfield on 16 Oct, 2022, 11:28
Ok so here are my preliminary findings:
I first of all looked at the HT circuit from slip ring to plug cap and everything looked good
My next test was to check the circuit across the points - there was continuity whether the points were open or closed
Resistance test between armature and body (earth brush removed) - gave a reading of only about 1.5 ohms
My thoughts are that with little resistance across the magneto, the bearing insulator is no longer effective, and a strip down is needed to explore things further.
Happy days
The armature primary resistance is quite low, around 0.5Ω, so difficult to measure and you will see little difference in resistance measuring across the points with a digital multimeter between points closed and points open. The armature should earth through the earthing brush, located on the side of the magneto. Check that it is intact and making contact with the armature. The bearing insulator is to prevent the armature earthing through the bearing as sparks could damage the bearing surfaces.
Thanks Derek. This is unknown territory for me, so I've been working through the 'Brightsparkmagnetos' check list provided by Bess. It says with the earth brush removed and assuming I have a fibre pinion drive, the body to armature shaft resistance should be over 500 ohms. I have around 1.5 ohms. Hence my thoughts re bearing insulators. Superficially everything looks good, but the magneto suddenly failed with no warning after being totally reliable.
My mag on the B33 stopped working some years ago and this turned out to be the bearing insulator. I think this started after the mag got soaked with rain water. Although the mag dried and worked I think the insulator broke up when it got wet.
I didn't think the bearing insulators had any effect on the functioning of the mag except to protect the bearings, as stated by Derek. You should still get a spark even if the insulators are toast.
No nor do I. Once I rebuilt the mag with a new bearing sleeve it all worked. Maybe the movement on the bearing cause the problem.
Could be. End float varying due to bearings movement would affect the points gap ( I think)
Quote from: Greenfield on 16 Oct, 2022, 14:46
Thanks Derek. This is unknown territory for me, so I've been working through the 'Brightsparkmagnetos' check list provided by Bess. It says with the earth brush removed and assuming I have a fibre pinion drive, the body to armature shaft resistance should be over 500 ohms. I have around 1.5 ohms. Hence my thoughts re bearing insulators. Superficially everything looks good, but the magneto suddenly failed with no warning after being totally reliable.
I have't looked at the brightsparksmagetos website but that sounds more like a check for the secondary, high tension, winding.
I had a lengthy chat with Andrew at Priory Magnetos this morning. He thought it was doubtful that there was a problem with the HT circuit and that sudden failure suggested that there was a problem in the points area, including no gunge down the hole where the securing screw goes. This could cause a short. Fully cleaned and back together, I notice there's a difference in the points gap as it travels over the cam plate. When the points open the gap is 9 thou but 180' further on before they start to close it's 12 thou, as set. Is this normal/acceptable and if so, do I set the points when they open or when they are about to close?
The points gap is set when they at the maximum opening.
Quote from: Greenfield on 17 Oct, 2022, 16:37
I had a lengthy chat with Andrew at Priory Magnetos this morning. He thought it was doubtful that there was a problem with the HT circuit and that sudden failure suggested that there was a problem in the points area, including no gunge down the hole where the securing screw goes. This could cause a short. Fully cleaned and back together, I notice there's a difference in the points gap as it travels over the cam plate. When the points open the gap is 9 thou but 180' further on before they start to close it's 12 thou, as set. Is this normal/acceptable and if so, do I set the points when they open or when they are about to close?
The points gap is not hypercritical. Generally I set them at 12 thou when they are at their maximum opening (as JulianS suggested). The variance in the gap could be due to the cam plate being slightly worn, or the cam plate housing not having been machined very accurately, or the cam plate housing not sitting perfectly level etc. This is old stuff which has probably been mistreated on numerous occasions in its life. The variation you've noticed won't matter at all provided they are opening and closing cleanly.
similar experience on my M21. Where the HT goes into it's housing was the problem. There was corrosion on the metal that made the contact. Cleaned it and off she went
Update
With everything cleaned to within an inch of its life and with points, plug and timing set as per book, I got a spark with the plug touching the fins. With the plug screwed in, it made one attempt at firing every 10 kicks or so. Fuel was getting through, but I felt it may be carb problems. The carb had been sitting on the bench for over a week so suspected that there may be crud dried up in the idle circuit. After stripping, cleaning and reassembling, and testing various combinations of advance retard, choke on and off and with or without throttle, I've finally got it running, albeit for short periods and as lumpy as hell with spitting out of both carb and exhaust.
I've checked the manifold for leaks and that seems fine. It used to idle smoothly at about 2/3 ignition advance, and when warmed up would idle at about 600revs fully retarded. I guess it's a matter of playing around with the ignition and carb settings until we get back to some normality.
Further thoughts
Having slept on the problem I thought that I'd start the day by checking the valve clearances with a cold engine. Both were fine. I haven't tried to start it again. It's raining and I don't want to have the garage doors open. Well, that's my excuse.
Going back to how the magneto works, very roughly with the cam diameter 3.5cm approx, this gives a circumference of 10cm approx. The cam depression is say 2cm or a fifth of one revolution. We set the points at 12 thou at the maximum gap. If that were 10 or 15thou it would mean that the points would open slightly earlier or later and be open or closed for a slightly shorter or longer period of time. Surely this minor change is insignificant compared with the crude changes brought about rotating the cam with the manual advance, perhaps around 10 or 15 degrees?
The timing is set at 3/8" btdc fully advanced. I think that most of us start the engine at 1/2 advanced then run fully advanced. So why have a fully retarded position if it's never used? Or is it to allow for inaccuracies when we adjust the ignition timing?
The plug gap is set at 20 thou. If it's say 15 thou the spark could happen a little earlier but have a shorter duration and the opposite if it's say 25 thou. Again, is this so critical with such a crude advance/retard mechanism.
So there we are, these old engines are by no means a sophisticated piece of engineering but mine is being particularly, let's say awkward at the moment. It's surely a matter of burning the correct petrol air mix at the correct moment to give maximum momentum to the piston at various revs/loads? During my investigations I've found nothing obviously wrong, but the spark has returned, and it's certainly given me food for thought.
What size gap will your spark jump. In the open you should be able to spark across 1/4 inch. If not it will be blown out under compression in the engine.
The plug gap will have no effect on the spark timing, that is entirely set by the time the points open. Too small a gap will and the plug will foul more easily. Too big a gap and the spark will try to find the path of least resistance which might not be at the plug gap. It is best to run as big a gap as possible to help to prevent plug fouling but .018" is normally best for a magneto ignition.
Thanks Derek, the handbook gives 18-20mm plug gap and I set at 20mm.
Re Idie's comment, the spark from HT lead to head is blue and clean somewhere between 1/8 to 1/4". As you'll appreciate, it's difficult to do holding the lead in the left hand away from the head, peering over the tank whilst giving it a brisk kick. So yes, perhaps a bit of room for improvement but should it work when not overworked?
A good way to check spark is to et an old plug and bend the side electrod right back to give a big gap. The plug can then be rested against the engine whilst kick the engine over.
Thanks Idie, it's another test but you can't get the electrode much further than 1/8" until it will spark sideways to earth, and you've got to assume that if you get a good spark here it will work ok when installed in the engine.
Before it cut out and ground to a halt the other week, what I used to find was after starting on full choke (hot or cold) it would run a bit lumpy with the air slide fully opened until warmed up. If it was already warm, I immediately opened the slide for best running. It wouldn't start if any throttle was used, hot or cold.
What I found yesterday was that I got it started on full choke, revs speeded up if the air slide was raised a few mms, but if opened any more it became very uneven and wouldn't idle at anything below about 1100 revs. It warmed up and as I played around with the air slide and throttle it cut out. The plug was very sooty on removal but yet it needed the air slide pretty closed to work. Screwing out the air control screw made things worse.
All very frustrating. You have got a non restive plug the same with the HT lead and plug cap. When ever I have your problems in the past it has always been caused by the mag giving a weak spark under load