BSA B50 Ignition Question

Sep 18, 2009
I have been working thru the wiring on my so called "restored" 71 B50. I have all the lights, turrn signals etc working fine. The Key switch positions appear to be the same as my Norton. Parking, Off, Ignition, Ignition plus lights. All that is sorted out and working well.

The bike has a capacitor which appears to hold a pretty healthy charge. I am dowm to the point of trying to get spark to the bike. My last two Nortons have electronic ignition so I am rusty in this area.

When the ignition is switced off. I still have some juice coming out of the capacitor and the yellow and white lead to the coil is still reading 12 volts. It will continue to read 12 volts even if the battery is disconnected.

When the negative side of the coil is still getting power this way, the positive side of the coil will still act as a ground and complete the circuit. The black and white wire coing down to the points still will read 12 volts on my meter.

The points do have the plastic insulators and I belive they are probably the original points that came with the bike in 1971. When the ground sie of the meter is attached to the arm of the points, I get a reading of about 7 volts. I should be getting a reading of zero volts.

Any thoughts / help would be appreciated.


Fiber washers under the points lever?
I let most of my unit single BSA books go some time ago. Then came into more unit single BSA's. Wouldn't ya know...
But I have one downstairs I could go look at.
I also have an exploded view from my Lucas book
it is an Energy Transfer system and is independent of battery supply
If you'd like I can scan what I have and post it for you.
Thanks for the offer. I bought a good workshop manual from Rabers in San Jose. It has a good section on the electrics. The first thing I did was redo the connection at the points. The wire was tucked under the nut and not isolated by the plastic isolator. I remember that much from issues with my Green Norton.

I screwed around for a few hours and was not making much progress. Finally I removed the black and white wire from the positive side of the coil. I ran a jumper wire from the positve side of the coil directly to the points. I ran another jumper wire from the positive side of the coil to the black lead of my meter. The red lead on my meter was grounded. When the ignition was switched on and the points were closed, there was no reading on the meter. When I opened the points I got twelve volts.

The black and white wire has to be pinched / shorted someplace.

It is an easy wire to bypass. I will make up a new wire and I should be able to see spark at the plug in a few days.