timing advance/retard lever

[Deleted User]

The magneto is a Lucas , rotates clockwise , with slack wire advance. I’ve just had the armature rewound with new condenser and slip ring , so hopefully OK ! ( but I’m starting to get suspicious) So I might take it out and bench test it. So I need to find some one local to borrow a carb from, I think there’s another member that lives on the Isle of wight.

[SwedScott]

Hi again

Yes, I must admit that it more and more sounds like a carburettor problem. I came to think about when I helped a friend to start his newly renovated BSA. With a newly overhauled AMAL carb it took rivers of sweat to get it started, and it wouldn’t pick up revs. In desperation the old tatty Gurtner (german AMAL copy) that was fitted when he bought the bike was refitted. And to our great surprise the BSA was running sweet as a nut!
You’re probably on the right track with trying to borrow a carb that’s tested and OK.
As one man told me: as long there is spark and petrol any old engine should start. And I myself could add: petrol in somewhat near right quantities!
I have never tried a Binks carb, so I can not give any advice about that subject.

Good luck with the experiments!
Leif

[Deleted User]

Cheers , but I think I’ll try your suggestion and look at the condition of the spark at fully retarded

[chris wastell]

Tman wrote:

chris wastell wrote:

My ’38 model only starts at full advance. I run it like that all time only retarding it to stop the engine. Cleverer men than I have recommended on here before, running at fully advanced all the time. So maybe your Binks needs adjusting?

If it were recommended to run always on full advance then wouldn’t Scott have fitted fixed timing ignition? You need to be able to retard it for starting and under load, like hill climbing.
If you retard it to stop the engine then the ignition timing “window” must be way out of sync with the movement of the cam ring.

My bike has a mark (on the handlebar lever) for full advance, and another for full retard, both marks taken from the movement of the cam ring, and it will run anywhere within these marks.

I have the carb set so it only just doesn’t idle when the timing is fully retarded. The ignition timing is set precisely as per Book of the Scott. The bike pull like a train up hills. I suppose its just a case of them all being individual.

[Deleted User]

Checked the Spark, good blue spark at full advance BUT as I retard the spark disappears after a few degrees. I think its because I’ve timed it ( full advance) at the tail end of the range of spark , if that makes sense. I’ll establish the range of spark on the magneto and set the timing , full advance, to the leading edge of this range. PHEW ! gobbledygook ??

[Deleted User]

Check where the brass segment is on the slipring when the points just open, you may have the wrong rotation slipring. Kev

[Deleted User]

Sounds as if either your internal timing is out or it’s a bit tired. Mags which are on the way out tend to need full advance to produce a spark, ones in good shape spark when partially retarded in my experience. Also they can be ok cold but lose their magnetism when hot sometimes. Remedy is re-magnetising. I once bought a “reconditioned” mag which was lovely and shiny on the outside but completely worn out inside. Taught me a lesson.

[Deleted User]

Physics check !

1 The primary winding’s charge the capacitor by cutting the magnetic flux ?
2 The secondary winding’s produce a high voltage to oppose the flow of current when the points open ?
3 this high voltage produces the spark when leaking to earth.

So is the position of the armature irrelevant when the points open to discharge the capacitor ?

[Deleted User]

Kev , my slipring has two slots, one that is about 4mm wide and one which is about 10mm wide I am assuming its the 4mm slot ?

[Deleted User]

I am sure there are other members who can explain better but here goes: as the armature rotates the low tension coil generates an electrical field which builds to a maximum twice per revolution. The points open just when the field is greatest; this causes the field to collapse and it is this sudden change in the field which generates high voltage in the high tension coil. The capacitor helps and also prevents sparking at the points. You will see that the relationship between the points and the armature is critical. Hope this helps, worth Googling it for more info. I am very happy to be corrected here by our electrical boffins if I have got it wrong.

[scott596]

Jonathan, it is the cam ring that has the two slots in, the 4mm slot is for the A&R cable and the 10mm slot limits the amount that the cam ring can move. The slipring is part of the armature and has the brass segments that line up with the HT pickups. Alan. scottworkshop@hotmail.co.uk

[SwedScott]

Hi

Interesting that you get no spark at retard. Something is definitely amiss. Even a “just working” magneto should produce a feeble spark when retarded.
A simple check of the magneto internal timing is that the breaker should open a little bit AFTER the maximum magnetic pull is felt, when rotating the armature. Never earlier and not spot on the maximum pull. For more information there is a good article in Scott Technicalities, written by Don Hewitt. But for a little check in the workshop the simple test above is all one needs. I have seen so many magnetos that’s just thrown together for selling, so I always assume that they’re wrong until proved otherwise!

Good luck with your investigation!
Leif

[Deleted User]

[attachment=1:14rsg4cx]IMG_0071.jpeg[/attachment:14rsg4cx]

Just to avoid any confusion I have added photos of my mag with the points just opening and the position of the slip ring which the brass strip is fairly central to the pickup, this is at the fully retarded position. This mag has an oil seal problem and is about to be refurbished which is why the slip ring is a bit black and oily, I’ve given it a quick clean so you can see the brass segment more clearly. Kev

[Deleted User]

Thanks again for all the help, looking at my slip ring vs TDC when position should firing occur when fully retarded , I’m looking at the brass strip on the slip ring to see if its still in contact ?

[Deleted User]

[attachment=0:3ho1z2pw]wrong2.jpg[/attachment:3ho1z2pw]
[attachment=1:3ho1z2pw]wrong.jpg[/attachment:3ho1z2pw]

Hi again,

At the moment forget about the engine position, just concentrate on the mag. Both pictures above , the slipring is in the wrong place. The one in my last picture post is correct position.

Yes the brass should be in the window where the pickup goes when the points just start to open, fully advanced and fully retarded. If not then when the mag produces the spark because the brass is not in contact with the carbon brush on the end of the ignition lead pickup then the spark will jump internally within the mag as electric is idle and will take the path of least resistance. It’s easy to check, remove both ignition leads and pickups. Turn the back wheel with the bike in gear and plugs out makes it easier so the engine turns in it’s normal rotation unti the points just open, if you use a mirror and torch you should be able to see in one or both windows and look at the position of the brass strip (whichever one its showing). It should be fairly central to the window, then try the opposite window with the points just opening on the other side of the cam, if you can see it just going out or coming into view that would be wrong. It does not matter where the advance and retard is sitting but check in both conditions the brass must be across the window not just coming into or going out of view.. Richard Blackburn has a video on utube showing how he sets the timing and may be useful to watch. Pictures can speak a thousand words. Just go to utube and search for scottparts. heres the link https://youtu.be/UQtXkGL0vIs Kev

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