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#838160 01/26/21 7:50 pm
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edunham Offline OP
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So early this year when I found that the capacitor and the battery on my BSA B50 weren't playing nice together, I took out the battery and my B50 became, by comparison, quite an easy and reliable starter. Fast forward a few months, and the B50 was on the bench for a couple of months while I adapted a 230 MM 4 leading shoe Grimeca brake to the front end. Last Sunday, it was ready for a test ride. Wouldn't start. No spark. My assumption was that I had disturbed a wire somewhere, so last night I started looking. Sure enough, the screw connecting the ground wire to the cap had almost completely backed out of the cap. Screwed it back in. Still no spark. Disconnected the cap and hooked up a battery. Spark! Connected the cap and the battery. Spark! Tried the cap alone again. No spark! It was too cold last night to start the bike on the battery and test whether the charging system is working, but the wires from the alternator to the regulator/rectifier showed continuity. So the theory for the moment is that the cap went bad. Question is: Would a shaky intermittent ground kill a cap?

Ed from NJ

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edunham #838161 01/26/21 7:56 pm
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You DO realize, I hope, that capacitors have a life-span.

Yours may have reached it's limit.

Usually about 10 to 15 years is that life-span, but they have been known to fail earlier.

edunham #838174 01/26/21 9:32 pm
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edunham Offline OP
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Thanks Irish. I did know that. I don't think this one was that old, Although these days when I think something happened about 5 years ago, it usually turns out it happened 10-15 years ago!

Ed from NJ

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edunham #838228 01/27/21 8:05 am
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Sounds like insufficient output from the alternator to me. You could try charging the cap up direct from the battery, give it 10 mins and disconnect the battery and pip the horn or put the brake light on. You should get something From the horn you won’t get much but you’ll get “a something” even if it’s it’s just the clunk from the solenoid. The back light would last a moment before going dim. This shows the cap is holding charge, but it’s not a battery and it’s not going to hold more than a seconds worth of juice. If this is the case then At higher rpm the charging system might be just sufficient to charge the battery but not start a bike on points.

If I’m right, then your next step is to check the magnetism from the alternator rotor.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

edunham #838249 01/27/21 2:19 pm
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edunham Offline OP
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Allan,
I might try that. If it was warmer out, I would just hook jumpers to the battery, start the bike and put a meter on it to see if it is charging. But your method sounds easier.

Ed from NJ

edunham #838251 01/27/21 2:32 pm
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To check the magnetism from the rotor, you would have to remove the rotor itself and see if each magnet would hold the weight of the rotor
Against something like a frame tube or the exhaust down pipe.

But checking the cap as I described will as you say be much quicker and tell you off the cap is duff or not. The cap I have on mine isn’t the standard 2mc Lucas cap but one which I think is a Much higher rating. Got screw terminals on it also so those are held with some ring terminals and serrated washers to stop them coming loose. Incidentally it fits perfectly in the Lucas spring fitting.

Last edited by Allan G; 01/27/21 2:32 pm.

Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

edunham #838345 01/28/21 2:28 pm
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edunham Offline OP
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Allan,
I tried charging the cap off the battery and it was a no go, so I have ordered a new cap. Thanks for the suggestion though. Incidentally, the cap that has gone bad was an aftermarket cap with screw terminals that fit into the stock housing. On unit singles that is a rubber cup rather than the spring used on twins.

Ed from NJ

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edunham #838406 01/29/21 2:01 pm
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If you are discussing the Lucas 2MC capacitor, their short life expectancy has been common knowledge for a long time. This has applications to the B50, but also the wider use in "isolastic" Norton twins that used them extensively.

Reference my 2006 article on the subject HERE.

Hope this helps.


Don't hide 'em, Ride 'em !!

RF Whatley
NE Georgia, USA

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