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BTH Magneto

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BTH Magneto
Started at 05:45am on the 6th May, 2018 by Count Johnny
Count Johnny Subject: BTH Magneto
Morning Chaps

Slo Poke (fitted a BTH Magneto) started second kick, yesterday morning, but wouldn't respond to the throttle - well, unless you count erratic running, frequent huge bangs and flames leaping out of the exhaust as 'response'.

Coil resistance (and remember this is the solid state coil that BTH supplies) is 0.5 ohms, which seems wrong, but I can't find a definitive answer on the interweb.
Can anyone tell me, for sure, what it should be? Thanks in advance.
Posted: 05:45am 6th May, 2018
Count Johnny Subject: BTH Magneto
Apologies for the above dogs' dinner of a post. hopefully it makes sense.
Posted: 05:48am 6th May, 2018
Count Johnny Subject: BTH Magneto
OK

I've done some more digging and it seems like 0.5 Ohms, on the primary side, and 2.2k Ohms on the secondary side, would be to spec.

So does anyone have any thoughts on the popping, banging and no power thing? To me, it seemed as if the spark wasn't advancing but - given that the BTH has an electronic advance curve - I ruled that out as unlikely.

Following the popping and banging episode, I changed the spark plug and he wouldn't start. Changed back to the old spark plug and he still wouldn't start.

The last time I rode him (Friday before last) he started third kick and ran perfectly, so I'm a bit at a loss.
Posted: 07:21am 6th May, 2018
Bullet Whisperer Subject: BTH Magneto
Hi C.J. - I can't help with info on the ignition components, but having ridden your machine myself in the wet, I remember a lot of misfiring, suggesting water was getting onto components and / or connections under the front of the tank - perhaps rhere could be a bit of corrosion lurking somewhere, causing a bad connection? Other than that, put your finger in the carb intake and open the throttle, to make sure the needle is still clipped into the slide and not dropped out,which can cause the symptoms you describe. Regards, Paul.
Posted: 08:45am 6th May, 2018
Revband Subject: BTH Magneto
0.5 is correct, if it was ok prior to fitting the mag?, I would be rechecking the timing.
Posted: 08:52am 6th May, 2018
Count Johnny Subject: BTH Magneto
Thanks Paul

The connectors are good but I'll try your suggestion for the needle.

Currently, I'm wondering whether the exhaust rocker is sticking as there was obviously a lot of unburnt fuel making its way into the exhaust pipe - and you may recall that this happened before (although, on that occasion, it bent the pushrod) and that the bore on Slo Poke's rocker oil feed pipe is quite small.

If that's the case - or if that's the best I can come up with - you can expect me asking you to install new rockers and a standard oil feed pipe (in a hurry, as usual).

Obviously, I'm hoping you're right about the needle thing. :)

I'll let you know.
Iain
Posted: 09:03am 6th May, 2018
Mark M Subject: BTH Magneto
Have you re-checked the timing? The symptoms sound like the timing might have slipped, I assume it's still a taper fit to the drive pinion? I'd also try phoning BTH for their advice, premature failure of new devices is not unknown after all, and I have heard of a similar fault on a new BTH fitted to a Velocette. (Reported on the RealClassic message Board.)

REgards, Mark
Posted: 09:19am 6th May, 2018
Count Johnny Subject: BTH Magneto
Hi Revband

The magneto has been on Slo Poke for a couple of years and is about the only thing that hasn't caused me some angst, in that time - so I guess it may be taking its turn.

Hi Mark

Yes, it's still a taper fit - and the possibility that the pinion had slipped was a theory that was discussed, with my pal, yesterday.

TBH, we got into the coil, because we couldn't see or hear a reliable spark, but it was very sunny, yesterday; getting the magneto spinning fast enough while holding a spark plug to the head has its difficulties on a Slo Poke; and (as it turns out) my mate's multimeter is manufactured, in China, by the 'Pi Lo Sh*t Company' and was reading 30 odd Ohms on the primary side.

Thanks both. I'll report back.
Posted: 09:59am 6th May, 2018
Mark M Subject: BTH Magneto plug check tool
A tip for holding the plug, get 2 smallish crocodile clips and bolt them back to back (they usually have holes in the ends for attaching a wire, a 3BA or similar nut and bolt will clamp them tightly together) and then clip one end round the plug and the other attach to a cylinder head fin. Perfect "no hands" spark checking! I read this recently and after making one wondered why I'd struggled for so many years!

REgards, Mark
Posted: 10:09am 6th May, 2018
Count Johnny Subject: BTH Magneto
Good tip. Thanks Mark.

Doing further research, bringing the kill switch wire into contact with the HT wire, can also send the BTH a bit mental - but, on my machine, this is cut very short (about 2") and insulated as I stop the engine in gear.
Posted: 10:30am 6th May, 2018
Adrian Subject: BTH Magneto
Both of my ASBO Bullets have the BT-H FM1R fitted. A restorer down in Dorset told me three years ago that these mags have no ventilation and can "sweat" internally when they get hot, but that the resulting condensation when they cool down can cause internal corrosion damage.

I didn't know if this was true or not, but I figured it wouldn't hurt to drill a small vent hole in the timing peg blanking screw via the screwdriver slot on ASBO12, and the mag is still fine, so I will do the same with ASBO14.

Keep us posted on this one.

A.
Posted: 03:28pm 6th May, 2018
Count Johnny Subject: BTH Magneto
Sooo...

There's still bags of compression; the needle is still attached to the slide; and the pushrods, rockers and valve springs all checked out; so we've either got a slipped pinion or an internal fault with the mag.

Given that it's the quickest option - and Slo Poke and I are off to Germany in two weeks time - I'm hoping that it's the pinion and will be giving Paul a call this morning.
Posted: 03:04am 8th May, 2018
Revband Subject: BTH Magneto
Unfortunately these BTH mags do have a bit of a reputation for failure.
Posted: 08:38am 8th May, 2018
Adrian Subject: BTH Magneto
Haven't actually heard that, though I did keep ASBO 12's contact breaker drive and Pazon Sure Fire "just in case"...

A.
Posted: 12:57pm 8th May, 2018
Mark M Subject: BTH Magneto
Yes, Adrian, see my earlier comment. This is not just hearsay, you can verify the story by searching for "Humbernut's" problems as posted.

REgards, Mark
Posted: 01:19pm 8th May, 2018
sofiaspin Subject: BTH Magneto
I had a BTH mag on my Velo Clubman and it worked perfectly. An old Velo pal was convinced it was witchcraft and could not see the spark. The spark is very strong, just difficult to see and if looking in sunlight, forget it. Sounds like it has not been set up properly. I am not aware of reliability problems and as you say, solid state, which has a lot going for it. Quality kit. Grove Motorcycles are the agents if I recall.
Posted: 03:10pm 8th May, 2018
Dennis C Subject: BTH Magneto
Yes indeed if you get a good one great.they do fail it would seem, yes humbernuts woes, I also have a friend who went through four failures before threatening legal action unless he got a refund, and yes he got one, personally I would not trust one on any of my bikes.
Posted: 04:12pm 8th May, 2018
Jamesy Subject: BTH Magneto
Recent problem similar symptoms caused by HT lead faulty.Spark was not a nice sharp blue constant but yellowish orange erratic flow.
Posted: 05:21pm 8th May, 2018
Count Johnny Subject: BTH Magneto
I've two posted lengthy diatribes on this, but the forum seems to have a timeout feature.

Just to clarify on one point. The BT-H has been fitted to Slo Poke for two years and has been jolly lovely in that time.
Posted: 05:32pm 8th May, 2018
Adrian Subject: BTH Magneto
Mark M,

point taken, no-one's beyond letting a substandard piece of kit out of the door, sadly, though despite the problem mentioned to me earlier I was previously unaware of large numbers of problems. I notice that some of their newer models now have external CDI boxes as well as external ignition coils, could this be a reaction to earlier failures? Count Johnny,

Further to Jamsey's point, racing HT lead (thicker copper core, yellow and black PVC sleeve) is sometimes recommended for magneto use if that's not what you're already using, though I'm not sure the current (sorry!) lead would be failing as dramatically if it has been fine up until now. While we're in that part of the system though, how's your spark plug cap? Might be time for a new one (unsuppressed as per BT-H's instructions).

A.
Posted: 11:05pm 8th May, 2018

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