Auto advance return issue

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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G3L1946
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Auto advance return issue

Post by G3L1946 »

Good morning,

Following on from my possible problems with ignition timing ( and damaged ankle! ) I took advice re checking the auto retard to see if it was sticking in fully advanced. The weights felt free enough but when turned 180 degrees it had a tendency to remain open when manually operated. Could this be just a clean and replacement spring issue or something more sinister. Replacement springs! Searching so far I have many options re part numbers ( looks like part number 498157 is correct for 350cc dynamo model ) is this correct? Also a tip on removal of mechanism would help. On undoing bolt it stops unwinding but unit and bolt remain secure.
Lucas SR 1 is the magneto. I believe mechanism complete part number is Lucas 47544. Photos show unit front on and half open position when manually opened.
Thanks, as always,

Steve
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g80csp11
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Re: Auto advance return issue

Post by g80csp11 »

best advice is to run the chain a bit slack as its less likely to stick in advance position
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G3L1946
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Re: Auto advance return issue

Post by G3L1946 »

Hi,
Thanks for reply. Chain pretty much as slack as I dare let it be I’m afraid.
P.S. Just took a look, checked and a smidge over 1/4“ play which, I believe, is about right.

Regards,

Steve
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clive
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Re: Auto advance return issue

Post by clive »

The nut is a combined tightening and extracting item. Can never remember how it works especially when putting it back. But fiddle with it turning this way and that and it should extract. If you are unlucky the pins holding the bob weights in place can wear and make it fail to return. Start with springs. I have never bothered with which engine size they were for.
clive
if it ain't broke don't fix
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G3L1946
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Re: Auto advance return issue

Post by G3L1946 »

Thanks Clive. I’ve discovered that the other option springs are for the 500 so I’m confident the ones I’ve in mind are correct. I’ll order springs and start there.
I need to remove the a/r unit not only to clean, inspect and change springs but I want to also get into check the valve timing as well.

Cheers,

Steve
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G3L1946
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Re: Auto advance return issue

Post by G3L1946 »

Hi ,
Back again! Having removed the a/r unit ( discovered that bolt also had a left hand thread? ) and thoroughly cleaned it and replaced with new springs I’m still having problems. When on the bench I can freely open the weights and, when I release them, they spring back shut.However, once back on the magneto shaft when manually opened up they stay open. Tried various tightness of bolt, tension of chain but to no avail. All parts are present, washer and back cup/retaining dish. I’m obviously missing a trick here or something is damaged or out of line.
Any ideas?
Regards,

Steve
Groily
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Re: Auto advance return issue

Post by Groily »

Suggest you go to Priory Magnetos website and look under the ATD section where much is explained v clearly. Might be fine, as loads of things affect operation in situ (- not operating the same as when on bench or with drive off is not abnormal).
Also, pix and explanations there will show you the left hand thread etc and generally demystify the auto-extract thing.
Springs used make small odds usually, worst thing can be if the main pillar in the middle is rusty / worn and there can also be probs with weights and pivot posts. But it takes a lot to kill them completely and they will often 'do' for a goodly while even when well past their best.
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G3L1946
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Re: Auto advance return issue

Post by G3L1946 »

Thanks Groily, will take a look.

Regards,

Steve
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jackstringer
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Re: Auto advance return issue

Post by jackstringer »

Groily wrote: Wed Jan 19, 2022 4:31 pm Suggest you go to Priory Magnetos website and look under the ATD section where much is explained v clearly. Might be fine, as loads of things affect operation in situ (- not operating the same as when on bench or with drive off is not abnormal).
Also, pix and explanations there will show you the left hand thread etc and generally demystify the auto-extract thing.
Springs used make small odds usually, worst thing can be if the main pillar in the middle is rusty / worn and there can also be probs with weights and pivot posts. But it takes a lot to kill them completely and they will often 'do' for a goodly while even when well past their best.
Link to source,
https://sites.google.com/site/priorymag ... authuser=0

Incase the Source is ever lost,
Priory Magnetos wrote: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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G3L1946
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Re: Auto advance return issue

Post by G3L1946 »

Hi Jack stringer,
Thank you for your reply. Very interesting information. The intro is my exact problem! I ‘thought’ I had cleaned the unit thoroughly. It occurred to me that as the problem manifested itself only when the unit was tightened ( ‘compressed’ ) there maybe some debris causing friction when in this state. I soaked it in some turpentine substitute ( all I had ) which, after a few minutes, went a brown ,rusty colour. I’m leaving it overnight in soak . But, it might be, according to the article, there may be no problem at all!. I will test it as it suggests after I re fit it.

Regards,

Steve
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