The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: oval57 on 25.04. 2016 10:22

Title: Ignition points
Post by: oval57 on 25.04. 2016 10:22
I have stripped my points out of the mag ( original brass plate type) and I read there should be some insulating washers in there, has anyone got a pic of the breakdown on how the points should be reassembled? Thanks
Title: Re: Ignition points
Post by: groily on 25.04. 2016 17:17
The insulating materials on a brass cb are:

* 'tophat' sleeve that goes down the hole in the middle with the wider 'tophat' portion sitting in a recess on the underside of the fixed point mounting block.  Accessible only when the fixed point is detached.
* sleeve that goes through the fixed point mounting block to insulate the small retaining screw from the block; there will be a small insulating washer under the head of that screw too.
* one or more flat insulators that go between the fixed point mounting block and the backplate. The centre screw and the small retaining screw both pass through it. The use of one or more of these enables the height of the contact breaker fixed point to be aligned correctly with the opening one.

There will also be a fibre or plastic pimple on top of the post that the moving point pivots on - to help the sprung arm keep things in place. But that doesn't do any insulating.

There is sometimes a fibre washer under the moving point, over the pivot post. That doesn't do any insulating either - but helps with alignment and reduces wobble on the pivot.

You can check that things are correctly assembled with the contact breaker off the magneto by inserting a bit of insulation between the points and then measuring the resistance across them. It should be infinite. With them closed, it wants to very very close to 0 ohms. (If you do this with the contact breaker attached to the magneto, you'll see there is only ever a very small resistance across the points, because what you are then measuring is the resistance of the primary, low tension, circuit in the magneto, which is about half an ohm.)
Title: Re: Ignition points
Post by: oval57 on 25.04. 2016 21:57
Thanks Bill, I haven't removed the fixed point I undone the screw that holds the long spring steel arm to the fixed upright spigot and something went ping, you know the story it's somewe're in the shed. I was curious as to what it is? Insulator or fibre washer? The lever that sits on top of that post that holds the moving points had what looks like a button made of rubber( looks like the waste button from when you use a wad punch ) would that be suffice? Bike simply lost spark and stopped, points aren't burnt only thing I am hoping is that the carbon brush on the back of that plate was stuck in ? Mag has been reco,d about 1000 miles ago so hopefully that side is good..... I will do ohms test on the points whilst I have them out and work from there..... Thanks again Roger
Title: Re: Ignition points
Post by: trevinoz on 25.04. 2016 22:27
Roger, the carbon brush on the back can be left out and the mag will still work.
How about the earth brush on the drive end?
The thing which went "ping" may have been the short helper spring.
Title: Re: Ignition points
Post by: oval57 on 26.04. 2016 00:15
Probably a dumb question but is that helper Spring required?
Title: Re: Ignition points
Post by: groily on 26.04. 2016 11:07
It'll work without, but it's there for a reason, so you should have one really! The bend on it should face inwards towards the centre but should not touch the brass block the fixed point is screwed into 'cos that would short out the low tension.
My shed floor is littered with unretrievable helpers and tits for spring arms and 6BA screws  . . . they always turn up, but not when you actually want them.
For a decent pic of all the bits (but ignore the condenser there which replaces the flat insulators mentioned above), see
Right hand side of the page. The helper spring is at 9 o'clock in case you ever meet it again when examining the floor!
Title: Re: Ignition points
Post by: oval57 on 04.05. 2016 22:29
All good thanks guys, all good now if you have read my "won't start" post the answer is in there, thanks again