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Discussion Starter #1
Finally got my '53 6T engine running sweetly.....I had to resort to a more modern brand new amal carb.

However now I can start it, and have it running, the next thing is being able to stop it!

It has a BTH KC2-S4 magneto....standard for this era of bike I understand...and I have the cut-out terminal connected to a 'horn' switch which goes to earth. I've checked it's earthing the terminal with a meter.
The terminal had the internal clip missing which made contact with the central nut, so I made one out of a feeler guage.

When I try and use this system to stop the engine....no success. I tried earthing the terminal to the engine casing with a metal rod (and got a slight electric shock whilst doing so) but it still wouldn't stop the engine.

Any possible suggestions?
 

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OK, I have to ask this, because I have seen this happen before... Do you have the mag and the bike charging system wired opposite polarity?
try unplugging your battery, and see if it stops
 

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Discussion Starter #4
Thanx for your replies guys......

Attached is a diagram of the cap parts to the BTH magneto...



I replaced the arm '63', which had broken off. Not sure of the parts you mean Paul?

As to the polarity of the battery, I'm running a 12v positive earth.
I assume the magneto is positive earth as the bike would have originally been positive earth.

I'll try disconecting the battery when I'm next in the garage.
 

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Discussion Starter #5
Right, I have now tried disconnecting the battery circuit....and this makes no difference.

I have also started the bike with the magneto cover off, and whilst the engine is running I shorted across from the primary chaincase cover (earth) to the central contact breaker nut (No. 58 in the diagramme). There was an occasional spark as I did this but the engine kept running????

I am now very confused.
 

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The battery/generator circuit makes no difference,because it's a separate and independent circuit.
I'm confused too.The centrebolt should be making good contact with the points plate.If you ground out the bolt,it should short-circuit the points and stop the engine.Is there anything stopping the centrebolt making good contact with the points plate,something that would insulate it or cause a bad connection?
 

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Discussion Starter #7
Thanx Mr Pete, I'm glad its not just me!
Below it a photo of the points with the central bolt removed.
The bolt is getting a good contact to it.

I tried again, and every time I short the bolt with a screwdriver I'm getting a spark. But the engine keeps going!

 

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Discussion Starter #8
I managed to speak to an extremely helpful guy who rebuilds these magnetos.

He immediately guessed the likely problem...and I thought I would let the forum know, just in case anyone else highlights a similar problem.

On the above diagram, I have the eathing brush and spring missing (No:56).....but you can get a similar problem if the brush is dirty or worn.

New parts are arriving soon and hopefully I'm all sorted!
 

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Methinks Grand Paul was onto that......it's an old trap. Seems to be a fair bit of old dried up grease behind that cam ring, might pay to clean it up & re lube.
 
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