Lights on, Lights off, Electrical conundrum!

[Barrieh]

BSA M33 Plunger - 1955 - New Lucas Wiring Loom - diagram followed religiously, new battery 6v positive earth, refurbed, Lucas Magdyno, new solid state rectifier.

Lights switch off / off from the battery, all good.
Run the bike, get it night are warm running for about 20 min, all good... except the lights have died....
Leave the bike for 30 min or more to cool down... lights come back on.

Its a standard off the shop floor 6v positive earthed system, the magdyno was professionally rebuilt... I am so confused... any ideas?

Any ideas warmly welcomed!

[Y13 Alan G]

Hi, The problem could lie with the rectifier. I'm not familiar with the M33, did it originally have a voltage control box or simple rectifier? If it originally had a voltage control box I have found to my cost with my pre-war BSA's that the modern electronic replacement needs to be well ventilated as if it gets hot it ceases to work and eventually stops working altogether. Hope this helps.

[Sigarus]

Hope that the solid state rectifier that you've bought is in fact a voltage control unit, as a dynamo doesn't need a rectifier (but alternators do). Does the ammeter show a charge with lights on and engine at moderate revs? If not, the problem might be simply that the dynamo is neg earth. This is quick and simple to fix - google 'how to reverse dynamo polarity'. If it isn't this then it may be a fault at a connection or failure in the control unit. It sounds as though your battery is getting flattened by the lights and then recovers a bit after being left standing.. Check what the ammeter shows with the engine running.

[Kuwait]

Also check the dynamo rotation is correct by making it motor.

[idie]

Try another battery. If a bridging bar has fractured it will open when the battery warms up then close again when cold.