SR1 Magneto Bent Armature?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
mrpjhancock_LAPSED
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SR1 Magneto Bent Armature?

Post by mrpjhancock_LAPSED »

i have recently bought a 1954 Matchless G80 with an SR1 magneto . . . . Yesterday I checked the Magneto chain tension and found it has a tight spot . . . . so I thought the chain has maybe stretched over the years so I replaced the chain. It still has a tight spot. So this leaves me to believe something is not running Concentric . . . Is it possible the magneto armature is bent? If so does anyone know if its possible to get it straightened? It starts/runs ok but the advance retard is stiff and I dont think its working properly cos its a bit sluggish. When I rotate the engine the bob weights on the A/R only spring back at certain positions . . . . again I think this is due to a bent armature. . Any suggestions please

Thanks
Mick D
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Re: SR1 Magneto Bent Armature?

Post by Mick D »

Hi

The symptoms you describe could also be generated by the sprocket not being correctly fitted to the mag shaft - you need to determine which, if either, is the case.

Do you have the tooling to measure the run out on the mag shaft, (DTI and support)?

It could also be caused by the 'other' sprocket not being concentric, (maybe just slackening the chain a tad could be a solution).

Regards Mick
mrpjhancock_LAPSED
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Re: SR1 Magneto Bent Armature?

Post by mrpjhancock_LAPSED »

I managed to change the mag chain with the magneto sprocket in place...... I didn't think of that one. I guess thats my next check. I can get DTI and stand from work and check for runout. What is the procedure to remove the mag sprocket? Thanks
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dave16mct
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Re: SR1 Magneto Bent Armature?

Post by dave16mct »

Undo the bolt, it should self-extract.
dave.
oldandsmelly
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Re: SR1 Magneto Bent Armature?

Post by oldandsmelly »

I think it's unlikely the armature will be bent for two reasons; firstly, the protruding axles are quite short and stubby and would be very hard to deform. Also, if the armature body itself bent even a smidge, it would start to whack the SR1 body as there is only a small gap. If your gauge does show run out on the shaft end, I think its more likely that one or both of the magneto bearings is failing.

The good news is that these bearings are available and not expensive. The bad news is that they are absolute beggars to remove as the inner races are a very tight fit on the armature and there is very little room to get a puller on.
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Rob Harknett
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Re: SR1 Magneto Bent Armature?

Post by Rob Harknett »

Not springing back is a common fault, they get rusty internally. They can be cleaned fitted, by pouring white spirit paraffin what ever on the unit and work it by hand You should see rusty fluid emerge. Just keep flooding and turning by hand until it snaps fully back when opened. Give a good spray of WD 40 as well as new grease in the timing case. Tight spots seem to be common on all chains.
Groily
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Re: SR1 Magneto Bent Armature?

Post by Groily »

With this acknowledgment, I'm pinching a bit of text I often quote from the www because it explains as well as ever it has been explained, by Andrew Guttman of Priory Magnetos, why ATDs don't always do what we think they should. It was written with rotating coil magnetos in mind, but much of the same applies with a magnetic rotor and static coil, as on an SR1. (Of course, things might just need - very often do -a good clean and oil, as Rob says.)

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.


As to a bent rotor shaft . . . it is possible but not that common. A few thou wouldn't be that unusual, but much more would be rare. And chains always have a tighter and slacker spots, even brand new ones. Ask anyone who replaces Norton cam chains regularly! If the sprocket isn't askew yet the problem is real rather than 'they're all like that', then either there is a shaft problem, or a bearing housing problem at the drive end maybe. To know for sure what is wrong would require removal of the rotor. Which isn't actually so hard on these - off with the points end complete with the contact breaker end bearing outer, and the shaft will come out. It's a good idea to slide it out into a close-fitting ferrous-metal tube if you can, to retain the magnetism on the rotor. With it on the bench you will be able to see fairly easily if the tapered section where the sprocket goes is bent, or if the problem is the bearing race being eccentrically positioned in the body (or loose even), which can have the same impact on chain tension. The commonest reason for eccentricity there is that the insulating cup behind the outer race is squiffy. But that isn't the easiest thing to get out unfortunately, unless things are already on the point of falling out! As has just been said, bearings are cheap and cheerful, but the inner bearing races on these aren't fun to get off without the right weapons. A perfectly workable tool can be made by drilling a piece of steel plate to the diameter of the bearing inner race, and placing round-nosed grub screws radially in threaded 'oles to grip the ball track, and hoicking it off with a fairly standard sort of puller. Outer races can be even worse, unless they literally fall out, as there's the square root of no clearance behind them to get 'em out. Worst case, two small holes through the body and up against the race, hard pins and a hammer will do it, but not a bodge for the faint-hearted. The alternative is a tool which will squeeze the race out by a 'camming' expanding action from behind, in conjunction with moderate heat.

With any luck, there isn't actually anything much amiss. Here's hoping!
bitza
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Re: SR1 Magneto Bent Armature?

Post by bitza »

For repairs try Tony Cooper over at Halesowen does a good job, not far from you.
mrpjhancock_LAPSED
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Re: SR1 Magneto Bent Armature?

Post by mrpjhancock_LAPSED »

Thanks for all your advice chaps. I've just taken both sprockets off ready to run a DTI check on them. I have to say looking at the taper on the magneto A/R sprocket it looks a bit rough . . . . like someone has been filing it in places . I suppose a replacement is a rare as Hens teeth?
mrpjhancock_LAPSED
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Re: SR1 Magneto Bent Armature?

Post by mrpjhancock_LAPSED »

Thanks for all your advice chaps . . . the problem was the fit of the A/R sprocket on the magneto amature. I cleaned the tapered bore up and smoothed off any raised points . . . replaced it on to armature and bingo! It runs true. The bob weights move freely and the bike goes better and ticks over evenly . . . .

Thanks again folks . . . . :D
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