A question about magnetos

OK, so the JAP I bought recently has a WICO A Type magneto, & it looks like the coil has had it, my brother had a Lucas SR1 (badly) fitted to to his Stuart Turner P5XC, this also had a bad coil (this one we had tested by a specialist, who confirmed it was shot).

Now, a quick word about these magnetos. These magnetos work on the spinning magnet principle, some others (usually older ones) work with a stationary magnet & spinning coil (these are the expensive ones to get re-wound & are generally a far more complicated magneto).

The other type of magneto which works by spinning magnet method is the flywheel mag, such as is used on the Tarpen, & on many old bikes, such as a BSA Bantam.

Now, a quick word about these coils. These coils, as has recently been discussed, actually contain 2 separate coils of wire, a thicker wire, of fewer turns, makes up the low tension side, where a much thinner wire, of many many turns, makes up the high tension side, both sides being earthed to the magneto chassis. This is much like a flywheel magneto coil (can you see where I'm going with this yet???)

Now, a quick word about prices. These basic magneto coils can be re-wound, but the cheapest re-wind is usually around £75. You can also get known-good old stock coils, tested & guaranteed for around 7 days, for around £50, you can also buy unknown old mags for anything from a few quid to in excess of £80.
But you can buy a WICO pattern BSA Bantam coil, brand new, made with modern materials & encapsulated in a way which is guaranteed to last for decades, for about £20, see here....



http://www.ebay.co.uk/itm/BSA-BANTAM-D1-D3-D5-D7-IGNITION-COIL-FOR-WIPAC-MAGNETO-/320969892825?pt=UK_Motorcycle_Parts&hash=item4abb4be3d9

So, why can't I get one of these cheap flywheel mag coils, put the correct sized piece of laminated metal core through the centre & jam it inside my WICO A-Type magneto???

I'm guessing things such as this definitely wouldn't work....



http://www.ebay.co.uk/itm/IGNITION-COIL-MAGNETO-ARMATURE-FITS-MOUNTFIELD-M150-RV150-SV150-V35-V40-/310423251802?pt=UK_Lawnmower_Parts_Accessories&hash=item4846aaeb5a

I think these come from breaker-less ignition systems, which don't have points (I think the other connection is for a stop switch, but not sure).

But why shouldn't the BSA Bantam coil work??? I mean, it has the low tension & high tension coils, there is a spinning magnet, & a bit of laminated bar isn't hard to come by, you may even get away with soft iron, although I'm not sure it can take the rapid change of magnetic flux like laminated can, but I've got lots of bits of laminated bar here & there, taken from various old motors, transformers & coils, I'm sure something could be shaped to fit, possibly even the original bar.

Can anyone tell me if this will or won't work, & give me a satisfactory explanation of why or why not? I'm sure if it would work everyone would be doing it, or maybe it would run, but isn't suitable for things like motorcycles, in which case it might be OK for a stationary engine.

Anyone know???

(Anyone know what I'm on about?)

Short answer:

It'll work - but...

If that's not good enough, carry on reading.

The last one first, the mountfield fitment - it's an 'electronic ignition' coil. It basically has a solid state set of points internally (probably a transistor) switched by the flywheel magnets. The small spade connector is for a stop switch, ground this to turn off. This wouldn't be suitable. One of the older style (with points) would work, if you can get the core out or modify it to suit.

The BSA one would be a better candidate, if the physical dimensions work out.

In short, a coil is a coil. There are a huge range of caveats to that statement of course but essentially, if it has an LT winding with an HT winding around it then it'll function just the same. Whether it'll actually work in a 'real-world' situation might be different though, say you have a coil designed for stronger magnets and yours are weaker you might not get a spark.

Depending on how much originality concerns you at this stage, you could even run an external coil for a while to prove the rest of the engine good? If you can do it without drilling holes etc. then no harm is done.

I wouldn't try using a solid core myself, if I couldn't get any laminate then I'd make some up with some tin-snips and varnish...

The bit you say about "if it works then everyone would do it" does give 'everyone' quite a bit of credit! For instance, I run my car on veg oil, it's as complicated as filling from a can, yet relatively few people do it. Why? Well, for a start every mechanic I spoke to about it said I'd be lucky if it carried on running for a week, and I'm 2 1/2 years on now. Also, look at what I did on that villiers using the original LT to power an external coil - I was told by a mag expert that it wouldn't work, I'd be wasting my time and that I should get a new coil for it.

Just because wisdom has been accepted it doesn't mean it's been accepted correctly or that it is indeed wisdom in the first place............

pauldg wrote:
Just because wisdom has been accepted it doesn't mean it's been accepted correctly or that it is indeed wisdom in the first place............

ie: the world was once believed to be flat ;D

I used to run my mk5 transit on 75% cooking oil in summer and 50% in winter with only possitive results, more power, greater economy and lower emmisions on the MOT. Unfortunately all that had to stop when I bought my mk7 as its common rail

Right, I've ordered one, I'll give it a try.

If it doesn't work I can always attempt to return it, or sell it back on & make "something" back, or just hang onto it, in the likelihood that I will have something with a flywheel mag which needs a coil at some point.

I've been offered a tested old coil for my mag for £50, which is a fair price, & comes with a 7 day warranty, but it's off an old mag, so could go on for ever, could fail next year. So I will gamble the twenty quid & see if this works, it may do.

Besides, I like a little experiment .

If it does work we can try one on my SR1 next and get that shifted

Right, I suppose I'd better write a bit about how this went.

The coil arrived on Saturday morning, whilst I was at the rally all weekend, but I got to work on my return Monday morning (well, Monday afternoon, I wasn't rushing about).

My main problem here was the size of the hole through the coil centre, it was quite a bit smaller than the original, so I used the Dremmel-type machine to grind a little of the rivets away so I could knock them through with a punch & remove some of the laminations from the bar. What I was left with was a reasonable fit, maybe a little loose but the hole in the coil is very slightly tapered & I didn't want to risk any damage. I wedged it in firmly with some of the leftover lamination pieces folded in half. Here's some pics of what I ended up with....









As you can see physical size isn't an issue, the coil is very small & fits in very well. I also soldered in my new 150nF capacitor as a condenser....





I only mention the condenser as I wonder if it has any bearing on my results.

I did manage to get a very small spark at the plug, but it wasn't there all the time & took some serious winding to get one at all, so all in all pretty unsatisfactory. Now I'm looking for reasons why???

There were two or three issues I feel weren't right, firstly my new laminated bar wasn't completely flat where it met the body of the mag (see pic 4 above for the joint I refer to, you may not see the gap, but there is one there, the bar did "rock" ever so slightly, despite my best efforts with a hammer, a file & even the trusty Dremmel-type thing with a grind stone on it.

Also, I wonder if my condenser may have some bearing on the result??? Will the smaller value capacitor make much difference.

Then there's the relationship between the bar & the magnetic laminations in the body of the mag, there's nothing I can do about it, unless I made up a soft iron bar, specially shaped to channel the extra available magnetic flux (again see pic 4 for what I'm talking about, the laminations on the body of the mag show the width of the magnets inside).

As it happens these coils aren't particularly well put together, the smaller wires are very delicately placed, having been badly stripped of their insulation & only the bare wire underneath is bonded onto the plastic outer of the coil, it was inevitable that one of them would come off during my tinkering (I wasn't rough with it either) so I'm sending it back as unfit for purpose, it's probably made in China anyway.

Any thoughts on weather this could be made to work??? Or is it simply a case of incompatible parts???

I'm guessing the fact I can only get a small proportion of the available magnetic flux to pass through the coil is the reason it is so unsatisfactory? I'm now faced with the choice of a known good, tested coil for around £50, or a choice of 4 A-Type mags which all spark, but are not tested, each priced at £40 but are a good 30 miles away, which will cost me a good £15-£20 in petrol to collect anyway.

Think I prefer the thought of a known & tested coil, as I know the seller "knows what he's doing" & indeed knows what he's talking about, when it comes to these things.

try a .22uf cap before spending bigger money.

I have to say I haven't a clue what your up to it's all a bit complicated to me, but I hope it works out for you and maybe you could help us all out with dodgy mags and make yourself a bit of money from it. I'd be happy to pay to get some of my mags back up and sparking again.

Stu.