BSA B40 1965 Technical question

Gecko · Apr 27, 2003

Sorry about this - I know it's not a GS question but it is technical and perhaps there is someone out there who also like to play with the old Brit iron sometimes whilst still being a GS rider too

My old BSA has a wierd problem so that when the engine is running the headlamp runs dim then dimmer and then fails. The horn goes from being a fair old 'TOOT' to become a stangled whimper.
When the engine is not running the headlamp can be turned on fine and bright and all the other electrics work fine - nice loud horn , brake light etc etc . The battery seems to be holding charge well enough. I tested it on my charger and it was still above at 80% and well in the 'green' so still adequate useable voltage. The problem only seems to be when the engine is running.

Does any one have any ideas ?
Cheers
Gecko

stan · Apr 28, 2003

i will ask a mate 4 you he builds them from scrap to show condtion

Rob Farmer · Apr 28, 2003

Gecko,

So a closet Brit bike owner?

The only BSAs I've worked on are A10s, A7s, A50s and B25s but it's close enough in the Brit bike world. The only thing I'm not sure about is if your bike was originally 12V. I'm assuming that all of your electrical equipment is 12V, and is fitted with an alternator, and that the bike has been working correctly up untill recently.

Have you put a multimeter across the battery with the bike running ?

You need to isolate the problem. The easiest way to do this is to disconnect the alternator wires. Then run the bike and see if the symptoms persist. if it does then move on to the rectifier, If the recitifier is the original selinium item there is a good chance that this may be breaking down to ground and pulling the battery terminal voltage down with it...if it is the rectifier I have a few modern ones laying about and I will send you one.

If the rectifier is ok then the only remaining item is the Zener diode but to be honest if a 6V Zener had been fitted it would pull down the Battery Terminal Voltage to 6V...my money is on the rectifier

Get in touch if you get stuck.

Squidbrain · Apr 28, 2003

1964 BSA. Alternator?

Phil Reynolds · Apr 28, 2003

I would check that the earth stap is making a good connection from the battery to the frame, and make sure that the earth strap is clamped to your battery securely.
The other thing to check is that your earth strap hasn't frayed.

Good luck

Isaac Hunt · Apr 28, 2003

1. is it +ve earth?

2. is the battry on the right way round or is the charging sitem trying to dischargwe it?

Gecko · Apr 28, 2003

Rob Farmer said:
Gecko,

So a closet Brit bike owner?
We all have our vices

The only BSAs I've worked on are A10s, A7s, A50s and B25s but it's close enough in the Brit bike world. The only thing I'm not sure about is if your bike was originally 12V. I'm assuming that all of your electrical equipment is 12V, and is fitted with an alternator, and that the bike has been working correctly up untill recently.
It is an original 12v system and the bike has been starting pretty much ok since the end of last summer. I road home one day in the rain last October and I guess that is the first time I was aware of the lights doing silly things - I just assumed it was moisture but it still persists after several months of dry winter storage and regular starting

Have you put a multimeter across the battery with the bike running ?
Uhhhh...? what does this do and what setting on the multimeter ? Am I just looking for volatge draining away or something else ?

You need to isolate the problem. The easiest way to do this is to disconnect the alternator wires. Then run the bike and see if the symptoms persist. if it does then move on to the rectifier, If the recitifier is the original selinium item there is a good chance that this may be breaking down to ground and pulling the battery terminal voltage down with it...if it is the rectifier I have a few modern ones laying about and I will send you one.

I think I have an alternator - I am sure I saw it in the haynes book somewhere . The rectifier is an original job so this could be a problem indeed....I'm not too well clued up on electrics. If there is no spark I can normally figure out the problem but this one is a 'head scratcher'....

If the rectifier is ok then the only remaining item is the Zener diode but to be honest if a 6V Zener had been fitted it would pull down the Battery Terminal Voltage to 6V...my money is on the rectifier
Zener Diode - sounds like something out of star wars Is that the little round thing with fins mounted on the frame ?

Get in touch if you get stuck.

Gecko · Apr 28, 2003

Squidbrain said:
1964 BSA. Alternator?

I think it's actually either 1965 or maybe even 66 (England world cup vintage year

) and I think there is an alternator in there somewhere....

Gecko · Apr 28, 2003

Isaac Hunt said:
1. is it +ve earth?
Yep - positive earth - according to Haynes book

2. is the battry on the right way round or is the charging sitem trying to dischargwe it?

On the wrong way round !!!! I bloody hope not I'd blow up wouldn't it ? I have ridden it several times last summer - up to 90 kms on one day no problems but now something is playing up.

ajd · Apr 29, 2003

I suggest you visit the message board at www.realclassic.co.uk and post a message there. My 1955 B33 is rather simpler, it doesn't have a low tension circuit to speak of...

Gecko · Apr 29, 2003

ajd said:
I suggest you visit the message board at www.realclassic.co.uk and post a message there. My 1955 B33 is rather simpler, it doesn't have a low tension circuit to speak of...

Thanks Andrew - I don't know that site at all but hace shot off a message and will see what comes of it. My missus will hate you for introducing mean to it because it means more hours of browsing the www

BSA B40 1965 Technical question

Gecko

Registered user

stan

Registered user

Rob Farmer

Well-known member

Squidbrain

Guest

Phil Reynolds

Well-known member

Isaac Hunt

Guest

Gecko

Registered user

Gecko

Registered user

Gecko

Registered user

ajd

Guest

Gecko

Registered user

Similar threads