B.T.H Mag-Dyno

[billirwinnz]

As I said in an earlier post on another "A" thread, Building an A (even a brand new one apparently) ain't for the faint of heart!

Nor is an old one. As Marcus said when I bought mine "Welcome to a lifetime of chasing oil leaks!". My first ride lost 500ml in 100km! It's much improved but I'm not there yet. My TTR is very dry so I can't see why a twin shouldn't be as well. I'm about to overhaul the top-end due to smoke, rattles and blow-by. I'm hoping to cure the last of the leaks (rocker box) after that is done.

 

[billirwinnz]

Getting a longer kickstart.
To get a bit more starting revs, seems a good cure.

Good idea. I happen to have a lengthened one. I'll try it.

 

[Robert Watson]

Having got the top end on the twin reasonably dry, I am about to swap the original cast iron heads out for a pair of new Conways ones. I am storing up patience at the moment and perhaps by the fall I might have enough to tackle them!

 

[billirwinnz]

Getting a longer kickstart.
To get a bit more starting revs, seems a good cure.

No luck. Same result. I've even tried ether. It will sometimes fire but won't start.

 

[Vincent Brake]

Lean on the front carbie?

 

[billirwinnz]

Lean on the front carbie?

It won’t even fire with ether. I think the problem is cold cranking speed. Carbs are from another A that starts first kick. Front carb jet block has been modified for bottom pilot feed (see Back to A) on both bikes.

 

[Martyn Goodwin]

I've recently fitted a modern BTH magdyno to my A twin. The bike idles and runs perfectly and starts easily when hot, but is very difficult to start from cold. I've experimented with timing and fitting the carbs from another bike that starts easily on a traditional magneto without any improvement.

The BTH instructions say that I should see and hear a cracking spark when the plug is held 1/4" from the head. I see this when running on one cylinder but never at kicking speed. It's possible to see a spark at kicking speed but only infrequently. I did try a test with pieces of paper between the plug points. No holes appeared unlike they do on my traditional magneto. Is that because of lower current and higher voltage not burning the paper or because I can't get it to spin over fast enough when the oil is thick and cold? It's winter here in NZ!

I'd appreciate input from anyone who is using a BTH successfully on their A.

When I had a modern BTH cdi kit on my Comet I had problems starting. Eventually I phoned the BTH folk to be told that the output below 250 rpm was zero, and it needed at least 400 rpm for reliable operation. Having full retard at 4 to 6 BTDC is also a big help.

 

[greg brillus]

Hi there Bill, do you know what weight of oil your are running, a heavy oil will make cranking the engine that much slower. For interests sake on Rodney's bike i temporarily installed a rebuilt original Lucas magdyno, and it made no difference to the starting.......The do seem to need a good kick for starting, my post war bike is no different so i'm use to it....... I feel an electric starter should have been part of the plan.........Hindsight's a great thing.

 

[Chris Launders]

Have you considered coil ignition, it is possible to retain the magdyno and use it as a trigger for coil ignition thus keeping the manual adv/ret.

I did this for a Brough until I got the pennies together to get a mag rebuilt, it's quite simple and doesn't involve any permanent mods to the mag part other than ruining a pickup.

You remove the slip-ring, all the windings and the condenser from the armature, fit a single cylinder slip ring and solder a wire from this where the HT input was to the connection where the condenser connected to the points, wrap the wire once round the central core and tape in place. Make sure you kill any flux after soldering.

I modified a pick-up by epoxying an electric drill brush holder in place of the original brush holder part (no longer HT of course) soldering a wire to the back first, this coming out where the HT lead fitted.

Then just wire up a twin output coil of your choice to a supply then that and a condenser to the above wire.

I built my mag up from spare parts but it still applies.

Chris.

 

[vibrac]

I was going to use the Thorspark ignition but found a guy who could sort what I thought was a duff magdyno for less although the system is comparatively cheap there were some negative comments on the web but I am never sure of the validity of net comments (on here is an exception )