The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: sckemp on 20.10. 2014 21:31

Title: Magneto only sparks on one side...
Post by: sckemp on 20.10. 2014 21:31
Hello, I've had my '51 plunger A10 for two weeks. It had been laid up for 10 years and today I got it running. It seemed to go well on both cylinders but it was also sparking through one of the HT lead pick ups onto the retaining tab. Then that side completely stopped sparking... The other side still sparks as usual. Points open fine both sides, leads and pick ups all have continuity through them, a multimeter test shows ca. 4000 ohms resistance between plug cap and centre screw on both leads indicating HT winding is good. I can't see why one side would work and the other wouldn't. Surely any issue would show up on both sides.

Some more info. The bike was taken off the road 10 years ago due to magneto issues. Mag was refurbed at the time and the owner at the time never got it back together again. When I got the bike, no sparks but I went through checking all connections, replaced condenser with easycap and shimmed to get the right end float and to get points gap equal on both sides. The result was fat blue sparks that jumped a 4mm gap from bare ends of both HT leads on turning by hand. So far so good. I installed the mag on the bike and set timing to 5/16 BTDC with ATD at full advance. Happy to say it was at 5/16 BTDC on both sides. Again so far, so good. Flood carb, couple of kicks, we have a runner! Then I noticed the sparks exiting the plastic housing of the pick up onto the retaining tab and after that, no more sparks on that side. As I say above, the connection through that HT lead is intact right through the HT circuit to the armature (I.e. points centre screw) so what could be the problem?

Any ideas? Thanks in advance (sorry - bad pun).

Title: Re: Magneto only sparks on one side...
Post by: groily on 20.10. 2014 21:55
Think you answered your own question. The HT pick-up from which the sparks were hopping out is leaking badly. If the spark is exiting there, very little or nothing will be getting to the plug. They do that when they're past it (some of the cheap pattern ones are known to do it from near-new) - and this problem would only affect the one side. Quick solution - all other things being equal and in good order - is to get another pick-up, or preferably a pair. Decent ones are handed, so if going for just one, you need to specify whether it's the rear or front pick-up (engine or gearbox side), and whether you want/need a 45° angle on the thing.
You could try wrapping the outside of the pick-up in a bit of rubber, or shoving a bit of inner tube between it and the clip to see if the arcing stops and the spark comes back on that side. Might not be successful if the inside of the pick-up is allowing leakage from the brush/spring to the mag body, but it sometimes/often is enough to prove the point. You may find the pick-up's surface has a slightly 'pumice-like' aspect to it in one area or another, where little sparks have been eating away at the material.
Cheers, Bill
Title: Re: Magneto only sparks on one side...
Post by: sckemp on 20.10. 2014 22:03
Thanks Bill. Makes sense - but is it normal for it to leak with visible sparks then for it to leak so directly that it doesn't need to arc anymore so you don't see it disappearing? Do you see what I mean? Anyway I will try with a piece of inner tube. Thanks again.
Title: Re: Magneto only sparks on one side...
Post by: sckemp on 20.10. 2014 22:31
Yep, that's sorted it! Will order some quality replacements. Sounds so obvious now but it's all part of my Brit bike education.