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Magneto timing

Submitted by david_cooper on Thu, 31/10/2019
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Nortons - General

First..a big Thank You to the Roadholder team (editor and contributors) for the magazine  and especially for the technical articles recently.

Which takes me to the excellent article on ignition this month, and especially on magnetos.  The notes about the optimum timing for the contact to open to match the collapse of the field leads to my question. Is the field collapse point on a lever adjust magneto optimised to match full advance to give best high speed reliable sparking  or on part retard to give easiest starting? Surely easy starting is most important as the mag speed will make up for a less optimal spark point?

Also... I understand the benefit of an auto advance sprocket is that the break point is the same anyway, so why would anyone go back to lever advance?

Thanks once again 

Regards 

Submitted by ian_soady on Fri, 01/11/2019

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the mag is set up for optimum spark intensity on full advance. This does as you suggest mean that on full retard the spark can be less intense but with a decent mag this isn't an issue.

I actually like manual control (especially with a long trials type lever) as I like to knock back to around 1/2 retard on approaching a roundabout or similar so I can stay in a higher gear. I developed this technique on a hand change Sunbeam I had but it's still quite useful with foot change, as is the ability to retard slightly at lowish revs on a climb.

Submitted by david_cooper on Fri, 01/11/2019

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Thanks very much for your comments, Ian.  But it does beg another question.  The book timing values for a prewar Inter can be over 40 degrees (I think 43 comes to mind), whereas most nowadays seem to agree that 37 is pretty much optimum.  I have actually set mine so the lever has a mark at 37, which is where I run it and it's fine.  But the lever has a bit of travel remaining.  So when I do retard it to start, I am even further from the optimum field collapse instant (if the mag cam plate was set up to be optimum at say 43 degrees).  Which might partly explain its reluctance to start!

Since it works at 37, I think I'll reset so I lose the option to go any further (now I know that 37 seems to be OK), and maybe that will improve starting.  (Although my experience of attempting minor improvements over the years is that they seldom make any difference!)  I've no idea how sensitive the filed collapse point is.  I'll look once more at all that data on the Brightspark site.

But it highlights the importance of having an accurately manufactured face cam plate.

 

Submitted by robert_tuck on Fri, 01/11/2019

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My 500 Rudge was impossible to start when I got it. I retarded the timing a little from the book settings and closed the points gap a couple of thou. I understand that this affects the internal timing a little. I can now start it without too much trouble. This is an old dodge used if the magnet is below par.

Submitted by robert_tuck on Sat, 02/11/2019

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When you think about it ,far more important to have the best spark to aid starting, not likely to want the best spark for max performance.  No start, no go at all!. 

Submitted by david_cooper on Sat, 02/11/2019

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That's what I thought. Don't know why they were designed the way they were. Now I know the timing I need, I'll retire so that is at the far end of lever movement. Life's too short to start experiments with different face cams.

Thanks for the comments.

Submitted by ian_soady on Sat, 02/11/2019

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the spark even on full retard would have been perfectly adequate. So it's not a design problem, merely the result of old age. Like my creaky knees.......

Submitted by robert_tuck on Sat, 02/11/2019

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I  retard the ignition just enough to avoid the  viscious  kickback,once found I mark the lever with a couple of dots of nail varnish ,colour optional, takes out all the guesswork and drama, although there may be a drama if you leave the lid off!.

 



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