Author Topic: A7 Magneto problem?  (Read 5189 times)

Offline a10gf

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Re: A7 Magneto problem?
« Reply #30 on: 16.02. 2008 11:55 »
^^ maybe my magneto lacks some orange and blue  *smile* ??
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bsa-bill wrote about adjusting the magneto realtive to case. Just some quick thoughts:

With a manual advance camring one can set the ignition time at any position, so I'd believe fitting a manual advance camhousing on any mag would give possibilities for fine adjustments without having to disassemble anything. Timing can then be done by replacing the cable with some adjusting bolt\thumbwheel. One would need a way of reading some accurate TDC \ timing marks, maybe finding a way to mark the points plate position relative to the outer edge of the camplate housing...? (Or, as I always did with old cars... adjust till pinking, then back of a little :O)

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Online groily

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Re: A7 Magneto problem?
« Reply #31 on: 16.02. 2008 17:38 »
Of course - obvious when someone tells you the answer! What is '7 C in the VS?' - typical pub quiz type question. Thanks Richard.
Re using manual cam ring and housing to enable tuning from the 'other' end, it's OK up to a point, pardon the pun, but one of the reasons makers went to ATD was to make it so that the sparks always occurred at the points of max flux of the armature .  . a manual cam ring messes with that. The spark weakens quite considerably the further you get away from the optimum. The effect is easily testable with a mag on the bench and a cam ring that is allowed to rotate by removing the peg; while a decent mag correctly synchronised will give you a spark that will jump 1/4 inch or more when you turn the thing by hand, as you alter the relationship between armature position and when the points open, you'll see the spark gets weaker the further out of position you go. So while a (very) few degrees is probably fine, or at least not harmful, it's definitely another of those things you can have too much of. With a manual ign system, the makers ensured mags were best synchronised at max advance, as that's where the cam ring is most of the time. Hard too believe that many early engines used the advance/retard as the sole and very inefficient means of engine speed control . . . so there has been some progress! Groily
Bill

Offline a10gf

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Re: A7 Magneto problem?
« Reply #32 on: 17.02. 2008 13:32 »
You are entirely right about the magneto's ' max spark' position. As I get it, the automatic advance ensures the spark always occurs at the magneto's ideal position, a manual advance moves the points opening away from the mag's max flux when retarding the ignition. With the adjusting of camring I was only thinking about the finishing fine tuning and maintenance of correct TDC advance, without having the hassle of removing timing cover and moving mag pinion\advance unit, the latter always (to me at least) seeming to involve a dash of luck to get it right when tightening up.

BTW, I have the manual advance, and end up at max retard for idling and max advance for driving, very seldom feeling any use for any intermediate positions.
e

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Online groily

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Re: A7 Magneto problem?
« Reply #33 on: 17.02. 2008 16:57 »
I reckon that's exactly and precisely it, fine tuning only, but I'm only a hobbyist!
Reason why I got interested in this was when a friend who knows far more than me broke down here in France with his Vincent Rapide (lucky man - to have a Rapide, I mean, not to break down far from home!) I got the job of trying to figure out what was wrong with it and pull the offending bits apart so he could order up and then come back when he had everything necessary. It was the mag, which had had its insulation melt internally and lock it solid.
He and I played endlessly with a rewound armature observing all these anorak things regarding points of max flux and effect of moving the cb in relation thereto. His bike has an ATD same as a latera BSA's; a Lucas KVF armature is exactly the same as a K2F in every way, what's different is the slip ring . . . and because it's a 50 degree V twin, one spark has to be delivered 25 mag armature degrees off optimal. That spark is noticeably weaker in bench testing, and explains why Vincent went to coil ignition quite early, although they didn't have the reliability of alternators to help them, just the dynamo from the good Mr Miller.
We wanted to see if things would be better with 2 sparks 12.5 degrees wrong, but that was hopeless because you can hardly get any decent spark much BEFORE point of max flux, which is what we were asking it to do on one cylinder, to allow the other to fire only 12.5 degrees AFTER. For reasons to do with Fizzix and things I don't understand, an armature is so synchronised at the factory that it delivers its spark about 3 degrees AFTER point of max flux - this lectrickery bends, it seems. While it will still deliver sparks as late as 25 degrees after, it is a poor creature at anything much before the optimal point.
Hence my diatribe about it only being feasible to play within strict limits if one wants to have a movable cam ring! Looking at that earlier post, I omitted to say that if one DID make an eccentric to engage the cam ring notch and give a bit of adjustment (which we did on this Vincent and it works brilliantly to allow minor post-setting correction), it's necessary to drill and tap the opposite side of the mag end housing, where there is a convenient spot, and put a grub screw in there. If not, the ring could start to wiggle out towards the points cover for the lack of anything to stop it.

Now, back to more mundane things in the barn, like the brakes on the wife's car . . . who said leave them be and hope for an insurance payout? Groily
Bill

Offline BSA500

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Re: A7 Magneto problem?
« Reply #34 on: 18.02. 2008 12:53 »
Right have now retimed the engine no pinking hurray.But first start this morning and again the left cylinder playing up %*@@****""!!!!.Wriggled the plug cap about and it starting running fine-ish again,so get to work 'borrowed' a crimp connector ring thing and wire that on,now it seems to be fine.So to recap broke carbon brush and at the same time engine mucks about so strip mag to remove brush-check,still mucks about,find plug misbehaving replace,still mucks about,find plug cap mucking about replace with crimp ring seems fine.Will let you know if dodgy spy sat in space lands on me because with my luck who can say *conf*

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Re: A7 Magneto problem?
« Reply #35 on: 18.02. 2008 18:28 »
And don't go under a ladder unless it's in a stocking on a leg! Groily
Bill

Offline BSA500

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Re: A7 Magneto problem?
« Reply #36 on: 18.02. 2008 20:05 »
Bike performed a treat on the way home tonight *smile*.I might even replace the other plug cap with a crimp connection and find a rubber boot to fit over it to make it waterproof.
Andy

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Re: A7 Magneto problem?
« Reply #37 on: 26.02. 2008 16:39 »
Right have now fitted the other crimp ring onto the timing side plug lead and further improvement, starts a treat and runs really well.Well pleased and rode the bike in the rain today and no damp issues.While I am on a roll I might even get given an alloy tank free of charge and a set of spitfire scrambler pipes and two reverse cones,then again I might not!!