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New Type BTH Magneto

Peter Holmes

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VOC Member
I would like to set up my twin with a steady heart beat tickover, it drives me insane sitting at traffic lights for what seems an eternity, only to have the bloody thing cut out the minute the lights turn to amber. If I remove one spark plug whilst setting up the other cylinder for preliminary settings do I have to be careful to earth out the plug not in use? This is on a new type BTH Magneto.
 

BigEd

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VOC Forum Website Moderator
I would like to set up my twin with a steady heart beat tickover, it drives me insane sitting at traffic lights for what seems an eternity, only to have the bloody thing cut out the minute the lights turn to amber. If I remove one spark plug whilst setting up the other cylinder for preliminary settings do I have to be careful to earth out the plug not in use? This is on a new type BTH Magneto.
If you are setting the timing statically it will probably do little harm if the plug/plugs are not earthed and in fact I don't usually bother. (It won't do any harm to earth them anyway. My Rapide has done towards 40,000 miles with a new BT-H fitted and without any ignition related problems.:))
If you are checking things at running speed any removed plug should be earthed. There is then no risk of you inadvertently becoming the earth and getting the shock. The high voltage will look for a way to earth and could look for an earth through the electronics where it might cause problems.
 

Magnetoman

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If I remove one spark plug whilst setting up the other cylinder for preliminary settings do I have to be careful to earth out the plug not in use?
The answer is yes, you do need to be careful. Under normal running conditions your plug fires at less than 5 kV so the HT coil never experiences a higher potential than that. However, if you don't earth the plug, in a "normal" magneto the voltage will rise to greater than 20 kV. That puts a lot of stress on the insulation of the coils, and if/when an arc develops between adjacent coils, or between the coil and the housing it creates a tiny conducting channel of carbon (from the burned insulation). That channel doesn't heal itself, and only will continue to grow in the future.

I don't know how high the potential gets one of the new BTH magnetos if the lead is left dangling, but it will be a lot more than 5 kV (unless there is some internal equivalent of a spark gap). Also, I don't know how robust the internal electronics are. Either way, you're asking for trouble if you don't earth the plug on any ignition system, including an older points/coil one, because you are putting a lot of additional stress on the system.
 

greg brillus

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On a small side note Peter, if you do remove the plug, whilst running the other, just bare in mind that flammable gases could easily be coming out of the open plug hole, especially if you have a spark plug hooked up to the HT lead and sitting next to the cylinder head, Don't want you or your pride and joy to go up in smoke.......;)
 

Peter Holmes

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VOC Member
On a small side note Peter, if you do remove the plug, whilst running the other, just bare in mind that flammable gases could easily be coming out of the open plug hole, especially if you have a spark plug hooked up to the HT lead and sitting next to the cylinder head, Don't want you or your pride and joy to go up in smoke.......;)
Yes, I had considered that, but I will have to discharge the spark somewhere using the engine as earth, I was hoping that with the spark plug out that there would be little or no suck/vacuum to pull fuel into the cylinder.
 

greg brillus

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You could always use an old plug and just tap the earth electrode down enough so that it is a dead short......No spark that is......and just rest it on the head next to the plug hole.....
 

Peter Holmes

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You could always use an old plug and just tap the earth electrode down enough so that it is a dead short......No spark that is......and just rest it on the head next to the plug hole.....
I am well out my knowledge comfort zone here, but surely if there is no spark at the detached spark plug then you are not discharging any built up current in the magneto.
 

Magnetoman

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...surely if there is no spark at the detached spark plug then you are not discharging any built up current in the magneto.
Note that the rest of what Greg wrote was "rest it on the head." You would be right if the plug were left dangling in air, but having it touch the head provides the necessary the path back to earth. However, it will want to bounce around, especially if the engine starts, so keep that in mind when finding a secure resting place.
 

greg brillus

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VOC Member
Sorry peter, unfortunately that is not the case.....the spark generated needs to go somewhere.....if it does not go to an earth, then damage to the coil may occur. In a standard KV-F magneto, if a plug lead came off by accident, the magneto has a pair of built in safety gap screws that allow for the spark to jump a gap ( about 6 to 8 mm) roughly and straight to the body of the magneto. Most modern sealed type HT coils don't have this feature, that I'm aware of, so damage could occur. So the output from the coil/magneto needs to go somewhere, either via a small gap to earth ( spark plug) or directly to earth.
 

Peter Holmes

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VOC Member
Sorry peter, unfortunately that is not the case.....the spark generated needs to go somewhere.....if it does not go to an earth, then damage to the coil may occur. In a standard KV-F magneto, if a plug lead came off by accident, the magneto has a pair of built in safety gap screws that allow for the spark to jump a gap ( about 6 to 8 mm) roughly and straight to the body of the magneto. Most modern sealed type HT coils don't have this feature, that I'm aware of, so damage could occur. So the output from the coil/magneto needs to go somewhere, either via a small gap to earth ( spark plug) or directly to earth.
As I said Greg, I am well out of my comfort zone with all things electrical, I understood the bit where I have to earth the plug, as I have done for the past 50 years when testing for a visible spark, the bit I did not understand was how a no gap spark plug would prevent any damage to the magneto without producing a spark.
 

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