B.T.H Mag-Dyno
- Thread starter billirwinnz
- Start date
Hi Peter
The bike runs and idles perfectly and once hot will start readily and idle on either front or rear cylinder. Tappets are all correctly adjusted.
For a long time I thought it was carburation as it would start hot but not cold. Friends with BTHs on post war twins had no problems but now I’ve learned that pre-war twins had lower kickstart ratios and the Oz replicas even lower due to belt primary drive. I believe that cold cranking speed is the problem especially as I’m not seeing spark holes in tissue paper placed between the plug points. Unless the cranking speed exceeds a certain value not enough power is produced to create a spark. Warm oil seems to be enough to make the difference.
Cheers Bill
The bike runs and idles perfectly and once hot will start readily and idle on either front or rear cylinder. Tappets are all correctly adjusted.
For a long time I thought it was carburation as it would start hot but not cold. Friends with BTHs on post war twins had no problems but now I’ve learned that pre-war twins had lower kickstart ratios and the Oz replicas even lower due to belt primary drive. I believe that cold cranking speed is the problem especially as I’m not seeing spark holes in tissue paper placed between the plug points. Unless the cranking speed exceeds a certain value not enough power is produced to create a spark. Warm oil seems to be enough to make the difference.
Cheers Bill
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If the starting is a big issue, you might have to revert to some form of coil ignition, and perhaps you might need to consider lowering the compression.......... I know this might sound drastic, but its probably easier than you think....... A change in pistons is not difficult.
Or remove the squish bands.
Perhaps do as i did on the ones i built.........run a 15 w 40 diesel oil..........much easier to kick over.........
I'm trying to catch up here.
Are we saying that the replica "A" Twins have the 19 tooth ratchet and matching quadrant? And, that this is turning the motor over too slow to start the bike with the BTh or Powerdynamo?
If so, how much faster would the "normal" 17 tooth ratchet and quadrant spin the motor? And would it be adequate?
[I only got grade 4 CSE maths]
Rivet counters of the world unite!
Regards, Delboy.
I spoke to Neal today about the gearbox kickstart gears.........I originally thought they used the larger 19 tooth ratchet gear, and the smaller quadrant to match.............But Neal said, no they used the smaller 17 tooth gear to speed up the engine cranking speed....... The primary ratio is higher than standard as well.........So if the factory twin used say a ratio of 1.7:1 these are more like 1.78:1 or similar. I have the pulley tooth number written down somewhere deep in my mass of scattered paperwork.
Thank you Greg.
It would seem that the "A" Twin might not spin over quick enough on the kickstart cold to suit a Bth or Powerdynamo, even with the "normal" 17t kickstart ratio?
That does seem rather strange for a brand new unit that is intended to replace a traditional magneto.
Is anyone in contact with Tony Harris; the man that designed and developed the modern Bth, about the theory of it all?
[He sold the company some years back and doesn't produce the current items, but the technology is probably still the same.]
Counting rivets, running out of fingers.
Cheers, Delboy.
I think the fact of the matter is the self generating magnetos today (really CDI units) were actually designed for singles, that can obviously spin over faster.........Kicking a V twin of moderate performance and compression is probably right on the limit of these units.........That was why on the new twins, the Powerdyno mags were converted to 12 volt coil ignition, thereby bypassing the original CDI unit and supplying a switched 12 volt supply to the ignition coil itself........Just for interest sake, some of these bikes actually run stock hemi heads, and/or had the squish band removed or at least modified closer to the stock hemi head........i'm not sure it really made a big difference to starting........Sometimes it actually comes down to a technique to suit that engine........just as many post war twins like to be started in a particular way.........As an example........A Comet that runs a Norton AMC type gearbox needs to be started different to the stock Burman gearbox due to the kicking ratio of the gearbox.........The Norton box seems to need the engine turned over a bit further for a good start.......Simply easing it just over TDC as per normal does not work from my experience.........All good frustrating fun............
A little snippet more folks.
Out of interest, I just found a note I made from a 2013 conversation with Tony Harris about his early Bth/Lucas Magdyno look-alike, saying that "It fires at 100 RPM, so should start okay".
None of the Vincent's I've ever worked on, have I ever needed to know how fast it spins over.
Okay people with a calculator or a maths degree, how fast do "A" Rap's turn over when kickstarting?