Mosin. This applies to conventional electro mechanical regulators. An electronic regulator needs the correct polarity from the dynamo to start with and a slightly different method must be used to avoid damage.
A new field coil is just that. Young and inexperienced. Old field coils have some memory and residual magnetism, enough to generate a small current which builds as the dynamo spins. This is missing from your new field coil. When you flash the dynamo you are giving the field coil a little bit of magnetic memory. Purists will realise this is all drivel, but for practical purposes all we need to do is give the field coil some initial magnetism by applying power directly for a short time.
First check the basics. Assembled and wired correctly, brushes making contact etc. Good earth connection to dynamo body. Regulator connections good and correct. Good battery. Start it up, let it idle and apply power to the Field Terminal for a few seconds with a wire from the battery live, while increasing engine speed above idle. Applying varying power to the field coil is what the regulator does to vary the dynamo output. More power here, the greater the dynamo output for a given engine speed. If all in order, ammeter will show output and lights brighten when you rev up. Doing it this way takes care of whichever way the field coil is connected. This initial boost gives the field coil and it's pole piece some residual magnetism. The technical term for this is "Polarisation", hardly gets a mention in Lucas Service Literature, and back in the day irate customers would return new dynamos to the service department only to be told to bugger off and give it a flash.
For an electronic regulator I'd suggest disconnecting the dynamo and simply applying the non earth battery lead to the F Terminal, again with the engine running. Then checking for output D &F joined, junction via load to earth. Finally check this output is polarised correctly, just to be sure. Plenty on the internet about how to do it and precautions to take with electronic systems where the wrong dynamo output polarity can ruin the electronics. The output polarity changes according to which way the dynamo is turning and which way round the field coil is connected. Polarising in situ takes care of this.
Usual Lucas wiring is one field wire and one brush wire to a common earth, other brush to D, remaining field wire to F.
The motor test joins D & F and applies power to this junction and the dynamo body. Good for checking continuity and basic operation, bit of a game if assembled into the bike and drive connected.
Swarfy.
