ET: Engine (Twin) How To Remove Kill Wire From Inside B-TH Electonic

[CarlHungness]

Has anyone ever dug into the back of a B-TH electronic unit? I want to dis-connect the kill wire, as I now suspect it could be the root of my starting, running problems. Have had a report that it was the problem on another man's machine, grounding against the HT leads.
I think mine may have gone duff, and shorted out my Amal kill button. I have had the kill wire dis-connected while testing this year, but is there a chance the kill wire has muffed up the innards? Any advice on how to proceed appreciated. I'll take the cover off tomorrow and see what I can figure out.

 

[Simon Dinsdale]

Carl
The reports of the kill wire causing problems are not due to grounding against the HT leads. The problem is IF the kill wire runs up the side of the HT lead then RF noise from the HT leads when the plug sparks can feedback down the kill wire and cause electrical interference with the electronics inside the BTH. If you use resistor spark plugs they will reduce the RF interference a little bit, but they will not eliminate it.

So if you have run the kill lead up the side of the HT leads then the easy test is disconnect it from your kill button on the handlebars and position the kill wire well away from the HT leads, I.e downwards and onto the floor under the bike. Then try starting the bike and if suddenly it runs ok then you have probably found at least one problem and only then can you look at removing or redirecting the kill wire.

I believe later BTH uses a kill wire that has RF earth shield built in just like a smaller version of the coax cable used for TV Ariel leads. Even with that though it’s still not good electrical practice to run it alongside the HT leads.
When I used a BTH I ran the kill wire under the engine and up the back of the gearbox and then into the wiring harness So away from the HT leads. To mechanically protect the wire under the engine I ran it inside a small diameter copper tube mechanically secured to the engine.

 

[Speedtwin]

Thanks Simon,
I have been pondering which way to route my kill wire which is coiled up behind the mag cover at present cable tied to the coils.
So far has not interfered with starting.
Al

 

[Ian Savage]

As much as I hate to disagree with my learned friend Simon, in the reported issue about this the cutout wire was wound around the ht lead.
I have had the cutout running parallel with the ht lead, in heat-shrink sleeving, for years with no issues twin and single.
BTH now supply the cutout wire screened to 'idiot proof' the magneto.

 

[CarlHungness]

Carl
The reports of the kill wire causing problems are not due to grounding against the HT leads. The problem is IF the kill wire runs up the side of the HT lead then RF noise from the HT leads when the plug sparks can feedback down the kill wire and cause electrical interference with the electronics inside the BTH. If you use resistor spark plugs they will reduce the RF interference a little bit, but they will not eliminate it.

So if you have run the kill lead up the side of the HT leads then the easy test is disconnect it from your kill button on the handlebars and position the kill wire well away from the HT leads, I.e downwards and onto the floor under the bike. Then try starting the bike and if suddenly it runs ok then you have probably found at least one problem and only then can you look at removing or redirecting the kill wire.

I believe later BTH uses a kill wire that has RF earth shield built in just like a smaller version of the coax cable used for TV Ariel leads. Even with that though it’s still not good electrical practice to run it alongside the HT leads.
When I used a BTH I ran the kill wire under the engine and up the back of the gearbox and then into the wiring harness So away from the HT leads. To mechanically protect the wire under the engine I ran it inside a small diameter copper tube mechanically secured to the engine.

Had the same kill wire for over 20 years, no problem. But possibly there was a hiccup and it caused internal grounding inside...it's the last resort for the B-TH...will try to remove the kill wire internally...I am out of options, carbs have been dis-assembled, sure seem OK..

 

[lee_812d]

Is the kill switch itself the problem though, maybe it has shorted inside somehow.

 

[vibrac]

When racing we just sniped the cable as it left the B-Th unit

 

[CarlHungness]

Is the kill switch itself the problem though, maybe it has shorted inside somehow.

It may have been the problem....last year. Couldn't start the bike..finally did so, it sat for months. I dis-
connected the kill switch, just on intuition...and have been trying to start it ever since with it dis-connected.
BUT, when I checked the kill switch just for fun this week, once I got the bike running I thought I'd re-connect
the wires..and as soon as I touched them the bike died..WHAT?? I was not pushing the button..it was three feet
away. The wires should just go together...So now I figure the kill switch (Amal Button) is bad, tested it and it has continuity , but also has cont when button pressed! So my theory is: Last year the kill switch was going duff, hard
to start the bike, may have 'fed' electrics back into the mag and made a mess...but, I have had the bike running without the kill switch three times, and can't re-start each time..get so frustrated I leave until the next day. Maybe the B-TH is putting out spark, but maybe not strong enough to run the machine...This scenario is driving me mental, so trying to keep a good attitude.

 

[CarlHungness]

Had the same kill wire for over 20 years, no problem. But possibly there was a hiccup and it caused internal grounding inside...it's the last resort for the B-TH...will try to remove the kill wire internally...I am out of options, carbs have been dis-assembled, sure seem OK..

If what you describe has happened, I have to wonder if it caused just enough damage to allow the unit
to spark a plug in open air, but not under compression. The B-TH bench tests ok, will spark if I hold it
against the clutch lever and kick, without the plugs in, but won't start the bike..and if it does, it just 'ain't '
runnin' right..and can't get it re-started. I KNOW the kill switch has not been disconnected in years..but last year
I couldn't start the machine..finally did..rode it to my new shop..let it sit. And out of the blue I dis-connected the
kill wire and began replacing the coils, new wires, bench tested the B-TH, etc..and when I did get it running
I tried to re-connect the kill wire with its connector..and when touched the bike died! Can't be..the button was
three feet away. Got it started again next day, and same thing..ran rough, tried to connect the kill switch, just
for a test, and it killed it again (almost..I just touched the wires)..and kept running..so the button is bad..and wonder
if the feedback it was giving last year muffed the insides of the B-TH. No visible damage, I took it off today and
took covers off, and all looks un-scathed inside, no burns..tiny black wire is kill wire and can't see how to dis-
connect...but am not using it anyway.

 

[lee_812d]

I don't know anything about the B-TH but I would think that if the kill switch was shorted last year then the bike would never have started unless just kicking it 40 times with it earthed would cause damage that would still allow it to start eventually. Over the years when stopping the bike with the switch it will have turned over many more times than 40. I assume with the kill switch connected now, it will never start (difficult to tell I suppose with it being difficult to start with it disconnected)
Anyway, at least fixing the switch is a simple task when/if you decide to reinstate it!