Quick electrical question: When ticking over should my B40 be charging its 6V battery (yes realise I should think about converting to 12V!)? The Ammetre is showing a negative charge (approx -2Amps)and I was expecting it to either be holding charge or adding charge to the battery on tickover. At higher revs I can get a positive charge (up to about +2Amps) Am I expecting too much!?
Only part of your alternator is in use without lights switched on and the discharge is probably just the current being taken by the ignition.
Lucas figures for an RM18 alternator with light switch in off position is a minimum of 2.5 amps at 3000 rpm.
Too much charge on the original wiring with no regulator boils batteries.
Probably nothing wrong with your system - does it balance when lights on when you increase revs?
My B40 doesn't show much in the way of a charge when riding. I've never checked the output but the battery doesn't go flat and always seems to have plenty of power for the lights and horn when not running.
The other question to ask is how accurate is the ammeter?
What has previously been said about boiling the battery has happened to a Bantam I owned. I ended up fitting a regulator rectifier supplied by Rex Caunt for about £30 which cured the boiling. The same regulator rectifier is suitable for the B40.
Just one last point, I don't think you can put a digital meter across the output - you just get spurious readings. In the owners manual there is some mention of having to put a 1 OHM resister as a load across the output to be measured.