Misc: Ignition BTH Magneto sparks erratically...

cybershoot

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Non-VOC Member
I have installed the electronic BTH magneto, the one that sparks with the two little coils and the electronic advance/retard mechanism. I'm super happy with this instrument in deed. My machine is a HRD Rapide Series B '49.
Testing the mag in the vice it sparks like hell at lowest revs but once installed at the flange were it is supposed to be fixed it sparks weak and erratically. It even refuses to spark at all. Earthing the mag off the machine it sparks again as it should. Does anybody knows how this might be possible? The Mag is actually earthed together with the crankcase ???
 

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Steve G

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Non-VOC Member
Had this exact condition on the Cannonball. Mag would throw a great spark when off the bike but erratic when on the bike. I sanded the mag flange and the case where it mates-up. Bolted the two clean, bare metal surfaces together and the mag performed perfectly from then on.
 

cybershoot

Forum User
Non-VOC Member
Had this exact condition on the Cannonball. Mag would throw a great spark when off the bike but erratic when on the bike. I sanded the mag flange and the case where it mates-up. Bolted the two clean, bare metal surfaces together and the mag performed perfectly from then on.
Thank you Steve - will try this. I'm pretty sure this will be the remedy! Did you continue to use the O ring?
 

Robert Watson

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VOC Member
Maybe I just got lucky. I have a BT-H on a Rapide. It starts first or second kick, the kill wire is clipped with the HT leads. And it is not just a fluke, it's been like that for several thousands of miles.
 

davidd

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VOC Member
Better still - get rid of the kill wire totally

My friend Jim Young was left high and dry when his BT-H stopped working. After getting it home he found that when he clipped off the kill wire and sealed it he left just enough to get it trapped by the mag cowl. It was just enough to ground out.

BTH Jim Youn 2015 TT 2.jpg


BTH Jim Youn 2015 TT 3.jpg


I am careful about the installation, but like Robert, I tend to do what I want, as long as it works. Sometimes what we think will work does not.

David
 

BigEd

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VOC Forum Moderator
My friend Jim Young was left high and dry when his BT-H stopped working. After getting it home he found that when he clipped off the kill wire and sealed it he left just enough to get it trapped by the mag cowl. It was just enough to ground out.
I am careful about the installation, but like Robert, I tend to do what I want, as long as it works. Sometimes what we think will work does not.
David
Some people have found a cure for erratic running by removing the kill wire others like Robert and myself have no problems with the kill wire clipped with the HT leads. (Mine has been routed together with the HT leads since I built my Rapide, 40,000 + miles ago and counting.) The kill wire is not essential so if you think it might be causing a problem snip it off leaving a short (well insulated) piece so you can restore the kill wire if that is not the cause of a problem. If earthing might be causing a problem Davidd's photograph of his magneto shows how simple it is to put on a good earth. Most things electrical work better with a good earth.
 

greg brillus

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VOC Member
Had a customer call in the other day with a BTH that was suffering a miss in the engine, found to be one of the terminals on one of the output coils that was loose. Once crimped up nice and firm, no miss no more............:)
 
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