I have just bought an old WD B40 converted to off road use. It has a direct ignition system (no battery), that I am trying to get to work. I have had it briefly running but no spark again now. My problem is that I am unfamiliar with this type of ignition. I shall be replacing the easy bits like the condenser, points and plug. I am hoping/assuming the alternator is OK because I did get it running for a few seconds. My main confusion is around the coil(s). Service sheet 813B seems to imply there are two. My bike has a normal looking coil and a thing that looks like a large condenser fitted close to it and wired in with it. I don't know if the coil is a normal one or a special type. I also have no idea what the condenser look a like is or how to source a replacement. Any help or guidance much appreciated.
There was available an option for off road use to have replaced the battery with this large condenser 2MC that lived in a big curly spring.
This 2MC soaked up the output of the alternator and stored the power briefly to fire the ht coil.
Make sure you're not trying to start it with the light switch turned on!
Otherwise it's quite conventional, alternator 2 wires output to little rectifier , output to this 2MC and earth , this 2MC to normal points and condenser.
Should go.
But ensure the lights aren't on as there's not a lot of output at kicking speeds, once it's going you can turn the lights on.
The B40WD is capable of battery less ignition by, as said, simply replacing the battery by a large 2MC capacitor that is then charged by the output from the rectifier in the same way as a battery. The coil shown looks standard B40WD but appears to have the small capacitor that normally resides next to the points mounted next to it.
Also as said, this capacity ignition system will not work if you any other electrical components powered up, like the lights. I have a vague recollection that there might have been a second small capacitor mounted on the B40WD next to the coil (I probably left it off when I restored mine) but that is probably there for interference suppression purposes only.
It's possible for the ignition system to work to a limited extent without the 2MC capacitor but, at best, this might only produce a very weak spark. If you do have the 2MC capacitor on the bike it would be connected to the output of the rectifier assuming the bike is fitted with one?
https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ebay.co.uk%2Fitm%2F313119747851%3Fmkcid%3D16%26mkevt%3D1%26mkrid%3D711-127632-2357-0%26ssspo%3DrL6WHwwNQX-%26sssrc%3D2349624%26ssuid%3DPXSNP9ymS5K%26var%3D%26widget_ver%3Dartemis%26media%3DEMAIL&data=05%7C01%7C%7C86a9b969406848e2f6ef08daf7fd61cb%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C638094964875485777%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=xWxU%2BmIxWiqBhe%2F0eJOd8xVUwJdOFZ7d%2FbW6CeCQb9k%3D&reserved=0
This link may work and take you to the correct wiring for the 2MC circuit
The wiring diagram for a capacitor ignition system using a 2MC capacitor (item #3) instead of the battery is shown below:
No 7 is the zener diode,
This is the "regulation" of the voltage output , stops bulbs blowing or a surplus going to the coil etc
Anything over around 13 volts went to earth, hence the fancy finned heating they were mounted in.
Many thanks for your help Gents. Much appreciated.
The above diagram for a capacitor ignition system is actually for a BSA B50MX - mine has this system with the 2MC capacitor supported on a spring to protect it from vibration and mechanical shocks.
Some B50MX's were also fitted with energy transfer ignition. This is basically a form of magneto ignition and requires a special alternator and a special coil. In practice, I think it was difficult to set up and to get to work reliably and the capacitor ignition system proved to be better although not without its problems.
In the capacitor ignition system, the 2MC capacitor is charged by the DC current from the rectifier and holds just enough charge to provide 2 A or so of current for the coil when the points open. However, the system is still a bit marginal and relies on the electrolytic capacitor to be in good condition. As the name suggests, this type of capacitor is filled with chemicals forming the electrolyte and are notorious for degrading over time. If you are going to add a 2MC capacitor, using a new one is the best approach as it's difficult to determine the condition of a used one without the right test equipment.
Thanks. I am guessing the the small cylindrical device close to my coil is a 2MC capacitor. It seems replacement ones are available from Brit Bikes on Ebay.
I think I'd personally start at the beginning.
You know it did start once , so can't be too far off right.
Start at the alternator connections, there may be 3 wires joined into 2 giving you 12volts.
Then where these connect to the rectifier and the output of that to its earth and supply terminals.
Then check all connections cleanliness and tightness.
Off course cb points condition and gap and coil connections.
If you have a multimeter, check the resistance of the screw on ht plug cap ideally 5k ohms , max 10k and no resistance in the spark plug.
Whip plug out
Ensure lights off
Give it a kick
Get a spark ??
Quote from: GeoffStovold on 27 Jan, 2023, 07:57
Thanks. I am guessing the the small cylindrical device close to my coil is a 2MC capacitor. It seems replacement ones are available from Brit Bikes on Ebay.
The 2MC capacitor is the order of 2" long and at least 1" in diameter from memory and the one shown in the picture looks to be similar to the small one fitted on the contact breaker points plate. You can gauge its size compared with a standard Lucas rectifier shown in the pic below.
Yes thanks. I have now found it, lurking under the seat wrapped up in Gaffa tape. I shall order a new one because the one on the bike is at least 28 years old. I am now wondering why I have two condensers. One in the contact breaker cavity and one adjacent to the coil?
Geoff
As long as you have 1 connected across the cb points to earth.
The other may be an extra army issue and used to suppress any ht sparks which would upset any radio equipment installed on the bike.
Normally the cb one is tucked inside the side points area or I'm the distributor,
As long as it it electrically across the cb wire to earth , you're fine
Quote from: GeoffStovold on 27 Jan, 2023, 10:20
Yes thanks. I have now found it, lurking under the seat wrapped up in Gaffa tape. I shall order a new one because the one on the bike is at least 28 years old. I am now wondering why I have two condensers. One in the contact breaker cavity and one adjacent to the coil?
Geoff
Looking closer at the parts book, there is a second small capacitor shown fitted on the bike that is similar to the one in the points enclosure. This would have been mounted close to the coil and been there to help to suppress radio frequency emissions from the HT system that could, potentially, have been detected by an enemy. As said, I think mine had the second one fitted before I restored it but I left it off as RFI suppression is not really required for civilian use :)
With any luck and a following wind and strong right foot, you'll be away