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Starting difficulties when hot possible ATD malfunction

Submitted by mike_chizlett on Mon, 09/04/2012
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Heavy Twins

I have a 1956 dommie 99

I've recently had my mag completely rebuilt and now the bike is reluctant to start when hot.

On rechecking the timing I've noticed that after tightening up the ATD onto the mag the bob weights become stiff and will only return, at most,half way. I'm assuming that whenthe engine is hot and things have expanded that the bob weights are keeping the timing fully advanced and therefore preventing the engine to start.

The bob weights move ok when off the mag

After the engine has cooled down the engine starts ok

Any advice on this matter would be much appreciated.

Mike

Submitted by gordon_johnston on Tue, 10/04/2012

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If the ignition was stuck fully advanced, you would get a mighty kick back when you tried to kickstart the bike. Is the engine completely dead when you try to start it when it is hot? If so, the likely suspect is a poor magneto. If you take the points cover off when the engine is running, is there sparking at the points? If so, it has a failing condenser (capacitor if you prefer). Pattern points need to be cleaned more often than Lucas originals. The tungsten tips oxidise and cause poor contact and massive sparking. Were pattern points fitted? Some old but genuine points might help. If you are lacking compression when hot, check the valve clearances. There is a small possibility of the carb getting hot and causing a fuel vapour lock which might just account for your problems.

Submitted by colin_sharpe on Tue, 10/04/2012

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Did the "Complete" rebuild of the magneto include re-winding the armature? This sort of starting problem is usually caused by a break in the winding. When cold, the wires touch, but when hot they don't. I had that problem on my Manxman, and went through the vapour lock and points ideas before sending the armature to independent ignition supplies for a re-wind. After that, the troubles went away.

Submitted by anthony_duffield on Tue, 10/04/2012

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Mike,

Not starting when hot sounds like the mag is at fault, have a word with Moathouse Magnetos.

My ATD last year did the same as you explain in your original question, it was a bit worn so I sent it to Dave Lindsley, he rebuilt it doing a really good job, on fitting back on the engine it did the same thing, after speaking to Dave about it he said that with a freshly remagnetised mag the springs of the ATD are not man enough to turn the rotor once you take the wedge out.

Taking Daves advice I timed the engine as per the book, boxed it up and since then it has run without problem.

I would take Gordoins advice and get your condenser replaced.

Good Luck

Tony

Submitted by mike_chizlett on Tue, 10/04/2012

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Gordon, Colin, Tony

Thanks for taking the time to reply.

I like the sound of the ATD springs not being strong enough to return the rotor. Perhaps new springs would be a good idea in any case.

The mag was completely rebuilt, rewound and new condenser fitted.

Yes it is completely dead when trying to start when hot

The engine has good compression and when started runs fine.

The mag does have modern pattern points fitted so I think the first thing to do would be to try an older set that I have albeit a bit worn. That said the bike performed ok last year when the mag was rebuilt.

Thanks again

Mike

 


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