Author Topic: A7 Magneto problem?  (Read 5188 times)

Offline BSA500

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A7 Magneto problem?
« on: 12.02. 2008 16:45 »
Today the bike has been playing up again after breaking its throttle cable has now decided not to run on one cylinder.I think I know the issue but just to check with you lot as well. Will not run on drive side cylinder well....
1.Swopped plugs no change
2.Swapped ht leads no change
3.Swapped pick ups no change
4.Checked valve clearences both cylinders fine
Rembered broken pick up carbon brush which I could not find *idea*.Would this if it was insdie the mag still cause bad arching and prevent consistant spark to the left plug(if it was in the slip ring area it would be nearest the left hand pick up).
 I am taking the mag off tonight to at least check and clean while I am there.If there is no brush then I am buggered if I know what is causing this ????

Online groily

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Re: A7 Magneto problem?
« Reply #1 on: 12.02. 2008 18:55 »
If there are bits of busted brush floating around inside, that's not good news, although i wouldn't like to say it would be more likely to cause a consistent fault on one cylinder than a general malady. Given that a mag can send a spark a good quarter of an inch, a bit of spare conductor in there could cause a lot of arcing. The good news is that there are sparks on one side, apparently all the time - so the mag works.
However, there are other possibilities, and if you see no sign of the missing brush (or its spring?) inside, if I were you I'd have a good look at the contact breaker. Are the points gaps equal for each cylinder, or near enough? A big variation spells trouble. The bearing could be floppy (maybe the insulator washer behind it has disintegrated - they can do that), the hole in the fibre heel that the moving contact goes on is oval, etc, etc etc. Less likely: - If you've got manual ignition, the cam ring could be sitting a tad cockeyed in the end housing owing to wear in the latter. Even if you haven't got manual ignition, check the cam ring is a snug fit in the housing - if there's any wiggle room, that can be bad in terms of gap variation.
Also worth checking the earth bush is there and in good condition, although the thing can run without it, I'm told, and that should not be responsible for a one-cylinder problem.
I'm sure all will be revealed when you have the thing in your hand . . . whatever it is, it probably ain't that serious. Clean the slip ring carefully if you take the armature out (might be covered in black dust from remains of old brush?) - and watch out for the 'safety screws' - if you don't take them out of the mag body first you'll likely bust the slip ring trying to pull the armature out, which is a big pain. (If you leave the armature in, clean the slip ring with clean rag on the end of something blunt poked through the pick-up holes.) Note the shims, if any, between the end piece that holds the cam ring and the body, too . . . need them to be right to manage the endfloat of the armature.  Make sure the points are clean and try to see whether they mate squarely when they are shut - they often aren't that good. Far easier to set their gap with the mag on the bench, too, as it's hard to get a spanner on the locknut and another on the fixed point when you're doing it with your head at a horrible angle and not much room, poor light etc. Fingers crossed. Groily
Bill

Offline BSA500

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Re: A7 Magneto problem?
« Reply #2 on: 12.02. 2008 19:54 »
I still have the spring so thats not floating about.I will strip the mag down(done it loads of times so the safety gap screws are the first thing removed) and clean it,had a look at the points and they are square and clean.Like you said setting and checking the mag is easier on the bench rather than rolling around on the floor will let you know what I find.

Andy

Offline BSA500

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Re: A7 Magneto problem?
« Reply #3 on: 13.02. 2008 09:06 »
The offending brush was inside the mag and had not been turned to powder so not too much mess.No bearing problems,shims etc all seems fine with the mag.So I shall take the morning off tomorrow to set up the mag and retime the engine.Fingers crossed.
Andy

Offline fido

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Re: A7 Magneto problem?
« Reply #4 on: 13.02. 2008 09:26 »
With the working cylinder spark plug removed, check for compression on the problem cylinder. If you have a compression tester you could compare the 2 cylinders.

Online RichardL

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Re: A7 Magneto problem?
« Reply #5 on: 13.02. 2008 13:32 »
Thinking along Fido's line, is it possible that the nib (I forgot the correct name) at the slide got inhaled into the cylinder? If so, the wire could be keeping the intake valve from closing, however,  the fact that the clearances are correct may kill that idea. Maybe there's damage to the valve or seat (we hope not). Maybe just having the nib with some wire attached jumping around in the cylinder precludes ignition. There is also the possibility that there was coincidental gasket failure at the same time (or nearly so) as the cable failure. I guess all of these are obscure, but it's interesting to speculate. I actually believe, and hope, for your sake, the problem will go away after the magneto service.

Richard
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Online groily

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Re: A7 Magneto problem?
« Reply #6 on: 13.02. 2008 16:52 »
If it worked before I reckon the offending lost brush is the culprit . . . fingers crossed again . . . Groily
Bill

Offline BSA500

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Re: A7 Magneto problem?
« Reply #7 on: 14.02. 2008 13:52 »
Ok,
 Mag rebuilt retimed and an attempt at starting,still playing up.Hmm said I.So checked the plugs again and then remembered that I had taken them out and laid them on the head good spark but had then put them back into the same cylinders as before and not swopped them as I had first thought.So stuck a spare plug in t'whole and away we go.Seems plug was failing under compression but I did have to retrieve the brush anyway so a good internal clean of the mag is never a bad thing.Of course now the timing is slightly out(advanced) and pinking.Bugger.
Andy

Online RichardL

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Re: A7 Magneto problem?
« Reply #8 on: 14.02. 2008 15:28 »
Well, you must admit, those of us who predicted the probelm would be corrected after the magneto was checked and serviced were correct, notwithstanding the fact that the magneto was not the actual problem.
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Offline BSA500

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Re: A7 Magneto problem?
« Reply #9 on: 14.02. 2008 16:19 »
Yep can't deny you're right.I have some photos to post of the bike so I should download those soon.
Andy

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Re: A7 Magneto problem?
« Reply #10 on: 14.02. 2008 17:20 »
It's always great when a problem's solved, no matter the bends in the road. And as you say, Andy, you had to get that brush out, as it was almost bound to cause trouble sometime. Boring and fiddly having to set the timing again, but hey, you know the thing's going to work now, so with light heart and light touch, and a wedge in the ATD to hold it in the full advance position (if you've got one on there), all will be made perfect. Groily
Bill

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Re: A7 Magneto problem?
« Reply #11 on: 14.02. 2008 19:57 »
Yes its got the auto advance its always a pain doing timing like this as you only know its right when you ride it and then if its wrong off comes the timing case etc again hey ho.
Andy

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Re: A7 Magneto problem?
« Reply #12 on: 14.02. 2008 20:31 »
'Tis a pain indeed. Have you thought of making a fixed length piston stop which goes through the remains of a screwed-in spark plug and stops the piston positively at the right distance before tdc? (Smash ceramic bit and earth electrode off old plug, shove threaded rod through, with washers/nuts etc both ends, and hey presto, a special weapon for the engine in question.) Have to measure as carefully as possible to get the length right, natch, but then at least you're down just to getting the points in the right place (and ATD wedged). Or you can make a rod which slides in the old plug, with calibrations marked by filed notches . . . stop turning when the right one lines up against a fixed reference point on the outside. Groily.
Bill

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Re: A7 Magneto problem?
« Reply #13 on: 14.02. 2008 20:34 »
I have one of those nifty screw in timing tools(Sammy Millers) which I mark tdc and then mark 3/8 inch above.

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Re: A7 Magneto problem?
« Reply #14 on: 14.02. 2008 20:59 »
Very nice tool to have Andy. . . had I but known they existed! . . . Should be poss to set it to rights pretty quickly with that little gem. Wish there was something to help with the other half of the job. Groily
Bill