charging question.

[Ray White]

Please could someone give me an idea of why my dynamo/cut out is behaving differently to the way it has always done in the past...  With the switch on full charge the engine is started but has to be revved quite hard before the ammeter flips over from discharge to charge and the light goes out.  When the engine settles down to tick over the needle flips back to discharge and the light comes on as normal.

(The car is 1930 with 6 volts.  The dynamo is a later Ruby type with a resistor added for Summer charge.  The CF1 cut out has been professionally rebuilt a few years ago.)