The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: warmshed on 08.07. 2012 19:32

Title: Magneto trouble
Post by: warmshed on 08.07. 2012 19:32
Hi, I am new to the forum but have been following it for a while now. I am trying to look after my Brother in laws 1959 A10. Recently It would not start, Found some water in the mag end cap and after a bit did get it running though not well.  It now refuses to start, spark looked poor but it has been running and starting easily.  I can get it running if I connect one plug (either) but it cuts out if I put the second HT cap on. Is this a common indicator of a fault type? capacitor, armarure, water?  Many thanks, in advance.   Dave.
Title: Re: Magneto trouble
Post by: beezermacc on 08.07. 2012 22:09
Sounds like the HT current is tracking round the slip ring, maybe as a consequence of dampness.  As the magneto has been full of water I would recommend removing it and cleaning it out. You may find other issues in there which need dealing with. After removing the mag, remove the safety gap screws, the pickups and the earth brush. You can then remove the cam ring housing screws and slide the armature out with the points still attached. Giving everything a good clean in a warm, dry environment might help.... but.......... As you are new to the forum it is worth stating that magnetos which have not been rewound and fitted with modern capacitors are on borrowed time; this issue has been visited a number of times so apologies to those reading this post and have heard it all before. If your magneto has not been rewound with modern wire and lacquer it is most likely that water will have penetrated the original coil insulation causing internal arcing in the coil and a resultant weak spark.
Title: Re: Magneto trouble
Post by: bsa-bill on 08.07. 2012 22:12
Water in the end cap can be condensation but elswhere in the maggie is not good.
Easy to check though take one of the pickups off so you can get to the slip ring, poke a pipe cleaner down in and see if it comes out wet,if not good clean the slip ring while your there and try it, if it is wet then the maggies going to need to come off and stripped
Title: Re: Magneto trouble
Post by: warmshed on 08.07. 2012 22:38
Thanks for the replies. The slip rings seem clean but I will check for signs of water. I have a good electrical background its just the symptoms I get don't seem to add up. Why should it start and run well on either cylinder but cut out when the second plug is connected? I would have thought if moisture is the problem it would not allow it to start on one cylinder. Ill give it a few days to dry out, Ill remove the pickups (new) to allow the air the circulate.  I was hoping not to have to remove the mag, being a lazy sort. 

 I run a Velo myself and getting rid of the mag was the first and best thing I did 30 years ago, runs and starts perfect every time.  Might recommend a change to electronic ignition if the fault persists. I know its not every ones cup of tea. Good charging system being the only requisite and is required anyway for reliable use of the bike with mag or electronic. 

Additional hints or advice eagerly awaited.
Title: Re: Magneto trouble
Post by: beezermacc on 08.07. 2012 23:57
 My theory..... the spark is choosing to track down the wrong plug lead when both are attached due to contamination of the slip ring. With only one lead attached there is only one outlet which, if the slip ring is contaminated, will take a spark from anywhere on the slip ring. Are you sure you have the leads connected to the correct cylinders? The firing lead is the one which exits the mag closest to the heel of the points as the points are opening.
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 00:42
Thanks, beezermacc,

I must have described the fault in an ambiguous way, the bike starts and runs on one cylinder ie on the left with the left plug attached to the left mag lead and  it will also start and run with the right plug attached to the right plug lead but both ways it stops when the other plug is then attached.

I don't think the leads have reversed themselves as one of the leads is shorter and cannot reach the right hand plug, but Ill check anything is possible when you have the things off and on a hundred times!

 I will try to clean the slip ring and see how it goes.
Title: Re: Magneto trouble
Post by: Black Flash on 09.07. 2012 02:47
Hi warmshed; You might get it sparking again by applying some heat with a blower type hair dryer. I've used this trick on several occasions including thawing a frozen air line on a semi truck.  If you decide to disassemble your magneto there is an excellent website with detailed procedure/instructions at
I recently did this to install their EasyCap condenser which was my first time for any type of magneto repair. Having step by step instructions with pictures made it much easier. They also have original Lucas workshop manuals for reference.
You will also need to learn how to set the timing if you haven't done this before. Hope this helps, Gary
Title: Re: Magneto trouble
Post by: beezermacc on 09.07. 2012 07:01
If you swap the leads where they screw into the pickups would they be long enough? i.e. unscrew the acorns and swap them at the mag, not at the plugs.  I still reckon its the slip ring!
Title: Re: Magneto trouble
Post by: Ted_Flash on 09.07. 2012 07:26
Don't forget the possibility of poor pattern parts - there are carbon pickups in circulation which are too soft and contaminate the slip ring after a few rotations.  If they will "write" on a sheet of paper they are too soft.  Originals have a groove running the full length of the pickup.  I believe Paul Goff has sourced some good ones.
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 13:56
Cleaned the slip ring but still the same, fires up easy and runs well with only one lead connected (either) but cuts out when second lead connected.  Put the left hand lead on the right hand cylinder and it starts and runs well (one cylinder).

So is the slip ring sectioned so it should only fire one cylinder at a time? or is it continuous so you get a wasted spark on the other lead?

Thanks. Dave
Title: Re: Magneto trouble
Post by: beezermacc on 09.07. 2012 14:11
The slip ring has a brass section which is about one-third of the circumference of the contact area. The magneto rotates at half engine speed so you only get the sparks you need. I use a dentist's mirror to check for cleanliness in the slip ring housing. It still sounds as though there is tracking around the slip ring. When you say you tried the left lead on the right cylinder, do you mean you swapped the lead entirely or you left the lead alone at the mag end and just moved it onto the other plug? If you have used the pattern pickups with duff brushes the slip ring housing will be full of dust which needs cleaning out.
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 15:18
Just seen a picture of a slip ring for a 2 cylinder mag, it has a brass insert to transfer the HT. As mine is starts with either lead on one cylinder the slip ring must be contaminated with conductive material. As I have changed the pickups I think Ted-Flash has hit the nail on the head. and the brushes  I have are too soft and have coated the slip ring.  I will try to clean it out again. and I shall give Paul Goff a ring to get replacements.  Will report back.
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 15:34
Beezer macc, yes I just swapped the right plug cap to the left plug and it started so it must be tracking.  I bit the bullet and have just removed the mag, There did seem to be some graphite on the slip ring.  I removed the armature and cleaned it the best I could with brake cleaner.  I suppose it could have damaged the slip ring where it has tracked, suck and see I suppose as its a bit of an ordeal to fit a new slip ring.  One thing I found strange is the drive cog was fibre even though it was not an auto advance unit. Haynes said use a puller or a drift but you cant get a puller in there. I can feel play on the tapered shaft on the end of the mag, took it apart again and it seems to be moving on the brass armature end so its not bearing end float. will have to dig deeper. 
The armature was clean and it looks like a new capacitor so if I can overcome the wobbly shaft I might be OK.
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 17:03
Oh dear, the wobbly shaft is the show stopper. new armature seems to be on the cards so at least my original problem will also be overcome.  Many thanks for those that replied and assisted me.
Title: Re: Magneto trouble
Post by: chaterlea25 on 09.07. 2012 20:18
Hi, All
I had this problem with my SR a few years ago, and have also found the same on other mags
There were poor quality sliprings being used/supplied some years ago
The "plastic" breaks down and becomes a partial conductor

I meant to try and run with a mag like this by closing the plug gaps right down to see if theres enough energy to spark both plugs???? never got round to it!!!!

To test the slipring for breakdown use a "megger" tester connected across both plugleads,
this quickly shows how good or bad the slipring is

Fibre gear wheel is standard BSA fitment.

I have also come across the loose shaft problem *sad2*

Pack it up and send it off to beezermacc   *idea*

John O R
Title: Re: Magneto trouble
Post by: warmshed on 09.07. 2012 22:40
"Pack it up and send it off to beezermacc" 

Thanks, its already winging its to him, very helpful on the phone and gave confidence that he knew what he was saying.
Title: Re: Magneto trouble
Post by: metalflake11 on 10.08. 2012 18:32
What is a "megger" test please?
Title: Re: Magneto trouble
Post by: warmshed on 10.08. 2012 18:50
A megger test is normally refered to a high voltage test of high resistance, normally in the 500volt range measuring in meg-ohms.  Its called megger after the most popular meter maker in the sixties, of this type of meter.   Care has to be used to choose a range that allows the correct high voltage for the item being tested. too high a voltage can indeed break down the insulation.
Title: Re: Magneto trouble
Post by: metalflake11 on 10.08. 2012 20:34
Thankyou Warmshed.