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BTH magnetos

mikelmonaghan

New Website User
VOC Member
In praise of the BTH electronic magneto
For at least two years I had been plagued by random hot starting problems and hesitant pick up on my Rapide. They began to get more frequent and although there was always a good spark when I checked the plugs and tinkering with the carburettor tuning sort of got me going after each starting failure I was forced to conclude that it was the magneto.
Rewind the existing mag. or get a new, non-original equipment, electronic system?
I bit the bullet and fitted a BTH electronic mag. Result – unbelievable! End of hot starting problems and much easier cold starting. Flexibility is dramatically improved to the point where I can take many first and second corners a gear higher than before and can actually trickle through town at 30mph in top. I don’t have to keep adjusting the chain because it no longer snatches as it used to. Fuel consumption used to hover around 40mpg – it’s now about 60mpg although I need to do some more miles to confirm that.
Incidentally, fitting and timing the BTH mag. is so easy that you’re left reading and re-reading the fitting instructions because ‘surely you’ve forgotten something’.
Does my experience match that of other converters?
 

peterg

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

Yes it likely does and probably explains no response to this post. The absense of any "confusion" surrounding a BTH install occassionally afflicting other retrofits wherein the successful few toss out regular/glowing praise to encourage others encountering problems mid-project to soldier on, results in little fanfare with these, defensive or otherwise. They just plain work, no generator/alternator required.

In my case, A BTH was at the head of the short list of tasks on the Red Rap in preparation for the VOC NA Rally in 2 weeks. I fitted this one last night. Why would one replace a perfectly good rebuilt Lucas and ATD after only 5k miles? When measured against the 1,200 miles aboard my Shadow after fitting a BTH in 1/09 it was a no brainer. The shortcomings of the Lucas mag design are well documented, innacurate cam ring machining requiring splitting advance between pots, etc.. so won't dwell on that. But, will say I'll take CD ignition any time available when a reliable spark is required in an overly fuel rich environment as typically seen with our Vin's and as Porsche discovered in 1968 with the fitment of Bosch MFI. The U.S. sourced Permatune CD box was the only solution, and a variation remained available pretty much up until the 964.

But, the real culprit here in seemless performance and even a predictable idle is the ATD. Like Vin shocks, a laudible exercise into the unknown then for which substantial evolution has only made its shortcomings more apparent now. Whereas most advance mechanisms must only adjust the mass of a short distributor shaft back/forth, this device must overcome the sustained load of a generator and control the mass of the armature on overun, with minimal leverage and small springs. Unless absolutely perfect, one can feel stepped feeding of advance on steady acceleration, and a blip of the throttle can be required to sling the weights so a little more momentary spring tension can snap the device back to retard. Speaking of springs, like /2 BMW's one has no idea what the advance curve has become between full retard and full advance settings when fitting non-original springs. As a BTH actually is set at full advance, and pulls timing out the lower the rev's , I have no idea if that analog map is ideal at every rpm setting but it is academic anyway - too many compression ratio's, cams, carb options for BTH to possible address for an over the counter solution - but what's delivered here is smooth and perfectly repeatable every time/over time.

I have no idea what fails on a BTH, a stockist here has spare coils, but short of ruining them with a torrent of water spray off the front wheel or suppressing them with too much resistance, don't know that they'd even be regarded as a noteworthy liability. Even if they only lasted 10k miles, it wouldn't be a problem, the blissfull 9,999 in between would have certainly cemented most one's affections for this device.
 

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Robert Watson

Well Known and Active Website User
VOC Member
I have one of these devices on my early B /166. I had planned on taking it as a backup to a Lucas unit on a long trip, but the bike suddenly went off -- would not idle down , ran very ragged and had starting issues - everything pointed to carburation. I had the NOS 1 1/16 carbs apart 3 or 4 times before deciding to swap out the mag. Well here is where you expect glowing reports. Not so. If you have early (read heavy sand cast cases) you will find the B-TH will not come close to slipping into place. 3 - 4 hours with the die grinder carving away very precious metal - all the time wondering how much further before I see the front of the flywheel assembly - very worrying. Eventually with some metal removed from the unit I could get it to bolt up square to the face without touching the crankcase where it shouldn't. Then it worked a treat, but alas did NOT solve the carburettor problem!!.. Eventually that turned out to be the end of one throttle cable nipple jammed out the bottom of the slide and not letting the slide to close fully, despite the nice clunk sound of hitting the stop (not) when released!

Good thing I didn't try to change it on the road!!

If you have early cases and are going to fit a B-TH watch out for this.

Robert
 

peterg

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

Dispensing with any thoughts of warranty return on said device, I'd be far more inclined to grind the mag flanges back than touch those precious crankcases.

Unlike a Lucas in which sealing is achieved between flange face and case with paper, these seal with the o-ring, thus some metal outside of that could be viewed as somewhat superflous/redundant as nutting down is only to resist rotation, not keep the device from being pried/blown off its seat, and not much is required for that....so now may the ammeter of glowing praise swing back to positive.
 

Robert Watson

Well Known and Active Website User
VOC Member
I think you missed out on where I had to relieve the cases - it was about 2-3 in back from the flange - right in the front face of the case - ended up in about 1/8"

Robert

See you in Cannon Falls
 

deejay499

Well Known and Active Website User
VOC Member
I fitted a BTH mag to the Prince outfit. The main reason was that to get to the points etc with the sidecar fittings and `D` crashbars was nigh impossible. I don`t think the starting is any better than a distributor and the acceleration is not as crisp, perhaps there is a different advance curve, or perhaps it`s just smoother. I like the reliability and how easy it was to install and set up. I would prefer a better set up to the allen key for the rear bolt that replaces ET179 as it is very tight and awkward on the Prince. Perhaps the case need trimming like Robert`s.
 

peterg

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

Starting? Doubt it's better than a distributor either - all previous comparisons made to the Lucas mag - and in fact, within those space confines for the generating portion of the instrument itself, you could likely exceed that BTH's low rpm output with a 6V Bosch "blue" coil on a decent battery but that's straying from the beaty of this unit. A compact reliable ignition source, with accurate timing adjustment and without need for a charging system.

Regarding crisp acceleration with your former distributor...if I may digress ad naus-ee-um for a moment here. That was all about the advance side of its operation and it likely was feeding alot more (maybe too much) initial advance in at low rpm than what you have now from the BTH whose mapping is likely more linear (analog so not programmable) as it relates to rpm gain. Example: an old centrifugal advance in a U.S. Porsche 930 (with the vacuum retard disconnected) had about 6-7 degrees steady advance on throttle tip in - and it felt sluggish - with essentially a flat sweep to max. My 930 here converted to EFI with completely programmed/integreted ignition timing has 7 degrees at idle. Using manifold pressure sensing in its map, on throttle tip in it can jump immediately to 20 if not 34 degrees without any rpm gain. As the efficiency of the motor as an air pump improves with rpm gain, ie, the cams, port velocity, etc. begin working etc. and the all important cylinder pressure rises, the high initial advance is wither taken out of it or ceases to rise at a rate comparable to that rpm gain. The net result is the same vehicle equiped in the latter format feels delightfully responsive and 500lbs lighter than the former in the lower rpm ranges.
 

vibrac

Well Known and Active Website User
VOC Member
Once again like the belt drive I have Bth on both my racers suddenly one whole are of worry has gone I know that the timing is right I just have to go the the other reasons if I am down on power.
I three seasons of the single and 2 on the twin I have not had an ignition problem. once the hidden allen key came loose and it is a swine to fit I took a long hard look and cannot see an easy way round it. I converted to twin sparks when I went to Methanol on the single at a cost of £15.00. Dont forget on the racers it doesnt hide behind a cowl and the front mudguards are skimpy to say the least and with the rain we have had in the last 3 years....
 

Hugo Myatt

Well Known and Active Website User
VOC Member
Hot starting problems

Having suffered hot starting problems with a 'rebuilt' Lucas magneto I am considering the BT-H route. I initially thought the problem was heat transference back to my Mk.11 carbs. These were fitted with special adaptors with metal to metal contact and they certainly became quite warm after a run. I reverted to adaptors with the rubber bellows fitting, with no metal to metal contact. Although these appear not to transfer so much heat they have not cured the hot starting problem. My query about the BT-H set up is the drive pinion. What does one use? The only solution I have heard of is to weld the ATD up solid. This does not appeal as it is irreversible and rather agricultural.
 

methamon

Well Known and Active Website User
Non-VOC Member
I fitted aBTH to the twin 4 years ago and have nothing but praise. My only concern was the substitution of one mounting stud for an allen bolt - not nice. From memory I avoided this by welding a nut to a socket; using this this required the removal of some metal from the body of the magneto (cant remember quite where) but the casting is very rich anyway.
 

peterg

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

There are two variations on the solid drive - which you definitely want rather than retaining the ATD which means buying the BTH with fixed timing forever wedding you to the former - using the same kit. One is a white nylon-ish gear which some claim grows too much with heat causing a lash/mesh issue and the fibre one that looks a lot like the old ATD gear (shiver).

I have one of each on my two installations and Tony's site illustrates exactly what you should purchase. In your case because of location, at VOCS. The fibre one was drilled a tad offset on some holes requiring dressing with a rat tail file to achieve alignment.
 

rapcom

Well Known and Active Website User
VOC Member
Plastic/Nylon/Tufnol ?

What's wrong with a steel gear? I know it is slightly more noisy, but I like the security of knowing that it will not "grow", will not shed teeth, will not absorb moisture or oil, and with such a light load, will probably never wear out. Just use a cam pinion with a suitable adaptor in the centre.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
BT-H Magnetos

The body shape of the BT-H flange fitting magneto enables it to fit on many different makes of bikes. Removing the rear fixing stud and substituting a socket head cap screw was the simplest and most economical way of making it compatible with a Vincent twin.
The need to relieve the casting/magneto body seems to be very random and is not usually a problem. The different thickness of crankcase casting I assume is due to variations you often get with sand castings. The weight of my '48 cases suggests there is certainly plenty of metal and the BT-H magneto went straight on. A "dry" fit first to check clearance after removing the old magneto is a good idea.
A fixed timing pinion (ET48), steel pinion boss (ET223/1), self extracting nut (E54), plus four fixing screws, nuts and washers are required. I got this kit from the VOC Spares Company. If you want to avoid buying more parts you can use an existing mechanical advance/retard pinion with the mechanism "fixed" by welding. [People who have had trouble with mechanical adv/ret mechanisms might suggest blowing up rather than welding up. :) ]
You can see some fitting instructions online at the link below. There are also links to photographs and fitting comments supplied by customers.

http://www.bt-h.biz/vincent_fitting.htm

Eddy
 

block

Website User
Non-VOC Member
More BT-H praise here

I installed a BT-H on my 1950 C Rapide at the NA VOC Rally. After easy installation (thanks to Neil Donovan and Tim Holcroft for the help and Glenn Shriver for the supervision) the Rapide fired 1st kick. It has transformed my machine (or at least the rider) and brought reliable 1 kick starting cold or hot, as well as stable idle. This all with clapped out carbs. With renewed confidence in my ability to positively start this machine at all times, I feel like I have a new motorcycle. Cheers to the BT-H.
Joe B.
 

mikelmonaghan

New Website User
VOC Member
BTH Mag

Great! I know how you feel about the starting - I had reached the stage where I prayed that no-one was watching when I started the Rapide - now it just happens.

I take your point about the clapped out carbs - mine are the original 276's and were almost impossible to tune - now I don't know why I fiddled about so much - they just seem to be stable now and I can even run them lean enough to require the air valve from cold.

Happy riding
Mike Monaghan
 

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