[Rick Parkington]

Good. One other thing, maybe worth running an extra earth wire from that frame earth screw to the crank case or the mag. - one of the nuts on the points housing is usually a sort of sleeve nut with a grub screw to nip a wire. That way the dynamo will be properly earthed rather than having to go through the frame.
And, hey, don't be too hard on CVCs they tend to get the blame for other problems and are more reliable than people think, they also benefit from being a proper automotive design and robust enough to cope with being ratted about in all weathers on a motorbike. Solid state stuff is radio-based technology and therefore more sensitive to vibration from engine and road shocks, damp - and battery misconnection. The main advantage is that they are simple and cheap so you don't need to know how they work - just plug in a new one if they pack up!
Cheers R     

[oggers]

The main reason for me checking polarity is indeed my desire to fit a DVR2 regulator in lieu of the prehistoric and quite probably unreliable mass of metal and wire that was on it originally. I intend to mount that in a glass case and call in Alice Roberts...

Continuty testing indicates that the negative wire from the battery goes to the earth post near the seat.   

[Rick Parkington]

AS Greybeard says it should be neg earth but I'd check how the battery is wired, in case it's been altered. A 1931 would originally have had a Lucas 3 brush dynamo which is regulated through the headlight switch and doesn't have a regulator so I'm guessing, if you are fitting an electronic unit, that yours is now either a later 2 brush or a 3 brush converted to 2 brush.
With a 2 brush dynamo and CVC (ie original Lucas mechanical regulator box) polarity is not too important. You won't damage anything however you fit the battery - it just won't charge if it's the wrong way, until you 'flash the field'. This is done by taking a wire form whichever is the 'live' side of the battery and crackling it across the F terminal on the dynamo.
Quick explanation for anyone interested, a dynamo creates current by rotating a coil within a field of magnetism. The magnet is energised by the battery, but not until it starts charging by itself. For this to happen there has to  be a little bit of magnetism permanently available to kick the process off. 'Flashing the field' sets the polarity of this 'residual magnetism' to  get the process going - so you're not trying to use a pos earth current to kick off a neg earth charge. This is a common cause of dynamos not charging - another is when they stand long enough for the residual magnetism to fade away. Then you have to start the bike and poke your live wire into F terminal until it starts charging, then the magnet should slowly build up its residue again.

If you get polarity wrong with an electronic regulator, most will blow immediately so I'd always check how the battery is wired rather than trust the wring is what it should be.
Cheers Rick   

[Greybeard]

Most pre-war bikes were negative earth - my 37 Sunbeam and 37 Ariel were. Post-war they gradually changed over to positive earth. I dare say they had their reasons. Ive changed my earlier bikes to positive earth simply to make it easier to swap batteries about with a common type of quick connector. I got tired of having and maintaining a seperate battery for each bike. Much simpler with just half a dozen or so batteries to keep an eye on and I reckon it prolongs battery life when they get used regularly.

Steve

[oggers]

Making sure again please - before I order a modern regulator

1931 model 9  - Reading about I understand 6V Negative Earth - is this correct?