Lucas ATD on SR1 magneto - several questions

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Lucas ATD on SR1 magneto - several questions

Postby Andy51 » Tue Feb 04, 2020 9:14 pm

Hi All, I have just refitted a cleaned-up Lucas ATD onto the SR1 mag on my '54 AJS 18S. I had a real problem starting it last autumn, and eventually traced it to a seized-up ATD. I dismantled it completely, cleaned it up, put in replacement springs, re-assembled and greased it; the moveable claw turned freely although it didn't 'snap' back, but turned fairly quickly. I put this down to a liberal coating of grease as it was better 'dry'. I tightened up the fixing bolt onto the mag armature, fitted the chain and camshaft sprocket (not yet tightened up) and rotated ATD and chain and obviously the magneto armature. In most positions it moved freely, but in some positions it was very hard to move the claw.
Is this caused by the magneto magnetism as the ATD advances/retards the timing and is presumably affected by magnetism?
If not, what might the problem be?
Should the mechanism 'snap' back or return gently?
Is grease the best lubricant? I have considered graphite powder lubricant.
I want to get this sorted before I tighten everything up and spend a happy hour setting the timing! Cheers, Andy

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Re: Lucas ATD on SR1 magneto - several questions

Postby Groily » Tue Feb 04, 2020 9:36 pm

These queries are quite common, and usually there isn't anything wrong. With full acknowledgement to Andrew Guttman of Priory Magnetos, https://sites.google.com/site/priorymagnetos/home -who is the author/owner of the following text, I'm pinching it because it says it better than I've seen it said anywhere else:

A bit of ATD theory!
ATD's can be a bit baffling! Occasionally customers phone me, whilst fitting their magneto, to ask if their ATD is faulty. The conversation goes like this... "I'm fitting the magneto and just about to fit the ATD. When I hold the ATD in my hand it flicks back OK but as soon as I start to tighten it onto the magneto shaft it doesn't seem to flick back properly. I can move it backwards and forwards by hand but it seems to have gone stiff and won't flick back on its own. I've taken it off again but I can't find anything wrong with it. What's going on?!"
Think of the ATD as a component that works in three stages.... 1) the fixed gear picks up drive from the engine then..... 2) the drive is passed on to a flexible connection made of springs and bob weights, then... 3) the drive is collected from the flexible mechanism by a nut fixed onto the magneto armature shaft. Remember that the gear is fixed because it is meshed to the camshaft gear, so the springs are trying to pull the magneto armature back to the 'at rest' position whilst one end of each spring is attached to the gear and the other end of each spring is attached to the armature shaft. Now here's the critical bit.... (and the reason why the ATD doesn't flick back when stationary) ... the magneto armature is not free-floating because there is friction and magnetism in the magneto which prevents the armature responding fully to the pull of the springs. The friction/resistance is caused by the pickup brushes, earth brush, heel of the points, drag in the bearings and the attraction of the magnets. As the engine is turned over very slowly the friction is reduced because the parts are moving - sliding over each other, and the springs are able to pull the mechanism to its 'at rest' position. You can test this by turning the bike over slowly on the kick start and watching the ATD return to its fully retarded position - Hey Presto!
ATD units are not very sophisticated but they do need to work properly. The springs should be strong enough to return the unit to 'fully retarded' as the engine comes to rest - this will enable you to start the bike again without getting launched over the handlebars. If the springs are too strong they will prevent the magneto advancing soon enough which may cause overheating and sluggish running. Springs which are too slack will advance the spark too soon causing advancement at kick start speed (kick-back) and spitting in the carb when blipping the throttle.

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Re: Lucas ATD on SR1 magneto - several questions

Postby Andy51 » Tue Feb 04, 2020 10:17 pm

Groily, thanks very much for this, it answers pretty much everything. Andy

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