B.T.H Mag-Dyno

[billirwinnz]

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.

Thanks Chris

I'm going to refit the repaired mag. I prefer not to have to depend on a battery.

Cheers Bill

 

[billirwinnz]

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.

Hi Greg

I'm using 20/50 oil. My old A starts first kick most times hot or cold on a magneto and the same oil. Fitting the carbs from it to the new bike has made no difference. I had the mag on the new bike before it failed and starting was better than the Powerdynamo or BTH but still not brilliant. I have since modified the front carb for bottom pilot feed so we'll see if that has made a difference. I'll refit the mag today.

The only other differences I can think of are possible kickstart pinion ratio or the squish head. Ariel fours with 4 small cylinders may not have needed a high speed start.

Cheers Bill

 

[Martyn Goodwin]

Thanks Chris

I'm going to refit the repaired mag. I prefer not to have to depend on a battery.

Cheers Bill

Possibly you should be talking to Neal to repurchase the unit you sold back to him? Get the dip-switches in the position for 4 to 6 BTDC for starting and the experience of others who have the setup is good. Just a thought.

ELSE either a Pazon (cause they are in NZ) or a trispark system - both give big fat sparks at even ten RPM!

 

[greg brillus]

The new twins had the same starting gears/quadrants as the square fours, the primary ratio is slightly different to an original "A" twin on account of the available pulley's that are available.........this also happens with the post war engines if running a belt primary.........so a stock twin has a ratio of 1.6:1 verses a belt that has a ratio of 1.66:1..........this will apply to the pre war twins as well........The squish band will give better compression efficiency and the comp ratio is higher than an original........The cams are based on the later prewar 5X cams.........these give the same opening/closing points as post war Lightning cams, but not as much lift on either valve.......i seem to remember the pre war twins had a primary ratio of about 1.7:1 not sure about the new ones.........Running a CDI type magneto as per the newer BTH or in fact the original Powerdyno's set up is the same.........that was why they altered them to be coil ignition rather than as a CDI because the cranking speed was too low.......carburation also played a huge factor in the hard starting issues, particularly the front carb float level.......this alone seemed to be a difficult one that no one had any idea about including owners of original bikes that to me, again did not know much about.

 

[Robert Watson]

As I read this a small flicker of light appears.

On My A twin I changed the ratio of the primary drive quite a bit. I made a new 53T clutch sprocket (which you can do on a Honda clutch but I don't think on a Burman) so I now spin the motor about 5 or 6% faster. If I recall I put at 22 and 46 on the final drive giving almost the same ratio as a post war twin.

I can't recall what I put in the kickstart gears but I did purchase a complete late Sq 4 box so likely whatever they had as standard. A certain machinist in town also made a couple of different ratchet gears to increase the kick over speed but I didn't need to change it as it does start reasonably easily.

 

[billirwinnz]

Thanks Greg and Robert
Obviously the post war bikes spin just fast enough to generate a spark as does my bike when it’s hot. From what you have said Greg, the belt drive bikes spin a bit slower than standard As.

Changing back to the mag is a mission on the A. Tank, exhaust and engine plates have to be removed and all the wiring changed.

Cheers Bill

 

[Robert Watson]

It took me about 3 days to put the BT-H on mine. It had a "guessed at" mag platform, which was OK with the Lucas but the BT-H required the whole setup to be correct, I recall a lot of filing, fettling and fiddling, and to think I took it as a quick swap if required when we attempted R66, I think 2013!

 

[billirwinnz]

That was 2 days faster than me! I had the additional problem that the Replica magdyno mount was set up for the Powerdynamo. To get the belt pulleys to line up and the magdyno to fit in I had to thin down the dynamo gears and cover. When I fitted the BTH I had to swap these over. I've just been through the reverse process. The flats on the BTH retaining nut are only 2mm wide and wouldn't take a socket. Loctite really grips on fine threads! Luckily my neighbour had a good Britool 7/8AF open-ended spanner that did the job so I didn't have to grind the chamfer off my nice Stahlwille socket. I was a little more sparing with the loctite this time.

 

[Robert Watson]

Hmmmm Yes, I recall Neal or Rodney telling me they went to a toothed belt. I'm still on bicycle chain! I had a bit of a time getting the slack all sorted out before tightening the sprockets on the taper.

 

[Marcus Bowden]

Robert my handsome your attempt at R66 was 2014 while I was doing the Cannonball with Ken Smith on a 1927 Harley J.D. then Harvey flew my bike over and we did it in 2016, what an experience. I used less than a pint of oil a day, Harvey was about three pints a day.