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BT-H Mag


New Forum Website User
Non-VOC Member
I have a BT-H mag on my Rapide which is about 5 years old and has covered 6 or 7 thousand miles.

Last night after about 10 miles I stopped for petrol after which the bike refused to start. On checking the plugs, which were new a month ago the spark seemed weak. The journey to the petrol station was ok, the bike started from cold second kick, the only sign of trouble ahead was a miss fire when pulling away from a roundabout a low revs.

I suspected an ignition problem, and after an hour or so of letting things cool down the bike started again. I pulled out of the petrol station and headed for home. After two or three hundred yards the bike dropped on to one cylinder and then stopped about another hundred yards later. I had to be carted home in the back of a van.

This morning I checked the timing which is exactly as it was set up. There is a spark but doesn't seem very strong. There is fuel getting to the cylinder but the engine will not fire.

On the final ride of last year the bike refused to start when warm, but I was at a friends house and after cleaning plugs we got it going and it got me home. I've been out three times previous to last night's outing this year but none involved stopping.

Has anyone experienced a similar problem with a BT-H or am I barking up the wrong tree? The bike is displaying the same characteristics as when the original Lucas mag was on its way out.




New Forum Website User
VOC Member
Hi My bike has a BTH and it died in the rain yesterday, taking a look at the ignition system i found the rubber sheaf that holds the Ht leads into the ignition coils had perished and cracked allowing water into the coil and the lead to come off the metal contact spike in the coil. The Ht lead copper wires sticking out were also covered in green oxide which would greatly increase electrical resistance (caused by previous water entry) I dried it as best i could and wrapped the whole lot in insulating tape and the old bike got me to the east midalnds rally and back no problem.

Peter Holmes

Well Known and Active Forum Website User
VOC Member
I also experienced this problem whilst on the Isle of Man at the rally in 2007, and it very nearly ruined my entire rally, it turned out to be faulty coils and apparently at that time, which would correspond to your five years of ownership this was a known fault. Whilst I was fiddeling with my non running Rapide outside a Douglas hotel a kind Vincent owner from the west country (Bernard) walked by, identified the fault and lent me his two spare coils that he always carried with him since his BT-H mag failed with the same problem, thanks Bernard, you saved my rally. In fact I rode home with the coils and then posted them back to Bernard. I then discussed the problem with the chap that makes the mags, his name escapes me, and he sent out replacements FOC, problem solved

Vincent Brake

Well Known and Active Forum Website User
VOC Member
Well here's some story: yust coming home from a drive Adam-Luxembourg and a planned drive there with the aermacchi club.
It (I dont dare name this apparatus any more) broke down on me, not due to wet or any other wireing reason, It was only 2000 odd
km old (12 months) spitting back like a turbine engine, no good. (not off timeing)
The unit before this also broke down on me, (leaving me in Italy)
I dont mention the broken coils, no harm done, but 2x times the CDI unit?????

I am trowing them in the bin and am going for 12 v electronic system with a spare near my B---ks, yust in case.
either Pazon or Dutch mr. Beck.
Now its up to Alton, Lucas?? (OK I will stick a spare (charged) battery as well.

sorry Tony, to have mentioned this, but something is realy wrong with them.

Vincent (brake) Speed

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Vincent, Have you thought about going for a D distributor, I went Electronic on one of my bikes, Not the same as you but if it goes wrong, Lots of trouble!!.My other twin has D type with a big 12v coil, I have had trouble with small coils, All you need for spares is a coil, points,and a condensor. All The Best Bill.


Well Known and Active Forum Website User
VOC Member
I agree with Bill on this. For a road bike the 'D' distributor, in good condition, has everything going for it and nearly all parts are small enough that a spare can be carried, (e.g. points, condenser, coil). If all else fails you can service it by the side of the road. And recon. units are available through MPH although I have no personal experience of these.


Well Known and Active Forum Website User
Non-VOC Member
Howdy Chaps,

BTH's - The mounting strap for the coils places them with plug wires forward and primary wires projecting rearward in very close proximity to the engine cases. Between engine vibration, flex in this mounting arm and a tendency (especially if less flexible thick copper core plug wires are substituted) for the cowl to push the whole plot rearward, you run the risk of rubbing the wires through and experiencing a short. I try to bend the pins outward slightly to provide running clearance for the wires away from the case.

Alan J

Well Known and Active Forum Website User
VOC Member
I'll stick with my old Lucas mag., for all it's faults-and I can always get J.B. to fix it for me!!-The other problem with proper coil ignition is that you need a good, reliable charging system!!-and they are "thin on the ground"!-I've probably opened the "floodgates" on this one!!
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