Just Classic Symptoms of a Failing Mag?

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Johnobirches
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Just Classic Symptoms of a Failing Mag?

Post by Johnobirches »

Thrilled to say that I've now actually ridden 30 miles on my 1956 16MS.

Starts ok from cold but not when hot - from reading the forum this looks like the SR1 mag is failing - no surprise but a bit frustrating because it looks like I'm going to have to send it away just when I hoped to ride the bike to its first section meeting and maybe even go on a run.

Just a quick question though (nagging doubt):- I rode it about 8 miles to fill up with petrol today - sure enough it wouldn't start afterwards so I browsed some second hand cars while it cooled. Then it started ok so I set of for home but it wasn't running right. Gradually the power faded 'til I had to pull over. It kept running but there was just "nothing there" certainly not enough to pull away. After a minute it got a little more perky so I set off but after only a hundred yards it went weedy again. I pulled over and tried to keep it alive by blipping the throttle but it gradually chuffed to a halt and wouldn't restart. Luckily I was near a friendly MOT garage and the owner let me pull in and even gave me a lift home as he had to deliver a car not far from where I live.

After a couple of hours I got a lift back there. The bike started first kick and ran superbly the few miles home.

My question is would a weak spark from a duff mag winding cause weak gutless running? I would have thought that if there was enough spark to cause ignition then a bang is a bang and it would either run properly or not at all. What's in the back of my mind is that when hot a valve may be sticking or a valve guide moving and causing the symptom.

Regards
John
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clanger9
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Re: Just Classic Symptoms of a Failing Mag?

Post by clanger9 »

Has it got any compression when you try to start it when hot? If so, that would probably rule out a sticky valve...
Also, it's probably worth putting a new condenser on. Cheap, and might be all that's wrong with it.
1989 Moto Morini Dart 350
1993 Ducati 750SS
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1608
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Re: Just Classic Symptoms of a Failing Mag?

Post by 1608 »

Yup! probably a couple of quids worth of condensor, fitted in minutes. Unlike the K2F mag where the condensor is buried inside.
Johnobirches
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Re: Just Classic Symptoms of a Failing Mag?

Post by Johnobirches »

Much as I hate to admit it to myself, it did feel pretty slack kicking it over on the roadside. I put a new Lucas condenser in it last month when I thought I'd cured a non-starting problem but it had just cooled down I suppose.

So If it's a valve problem I'm hoping its more likely to be a sticking valve as a slipped valve guide wouldn't go back when it cooled. You'd think though that things would get looser when hot not tighter/sticking.

I'll get a compression tester and get the bike hot again.

I appreciate the help chaps.

John

John
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dave16mct
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Re: Just Classic Symptoms of a Failing Mag?

Post by dave16mct »

Hi John,
Not your tappets a bit tight?
Dave.
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clanger9
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Re: Just Classic Symptoms of a Failing Mag?

Post by clanger9 »

Have you got enough oil going to the top end? If the feed to the inlet valve is too restricted, it could nip up when warm.
Just a thought...
1989 Moto Morini Dart 350
1993 Ducati 750SS
Groily
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Re: Just Classic Symptoms of a Failing Mag?

Post by Groily »

You didn't say whether the mag was actually producing decent sparks when it was hot? It should make sparks able to jump a quarter inch from the tip of the HT cable to the head (say) when it's hot, spinning the engine on the kickstart. If it's intermittent or the sparks are pathetic, then I'd replace the condenser as clanger9 suggested before starting to look for other problems.
Johnobirches
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Re: Just Classic Symptoms of a Failing Mag?

Post by Johnobirches »

OK plenty to go at here.

Thank you, it's much appreciated.

John
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clanger9
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Re: Just Classic Symptoms of a Failing Mag?

Post by clanger9 »

...and don't forget that even new condensers can go faulty!

Drove me half mad when that happened to me: replaced everything, "It can't be the condenser, I just replaced it last week!", etc... :headbang:
1989 Moto Morini Dart 350
1993 Ducati 750SS
Johnobirches
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Re: Just Classic Symptoms of a Failing Mag?

Post by Johnobirches »

Thanks Clanger - certainly add another level of bafflement doesn't it?

John
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