[Brian]

Moving the third brush down, ie in direction of armature rotation, increases the charge rate. Start with it positioned just slightly down, run engine and watch ammeter. You want it indicating around 5 amp charge. Move the brush up/down to accomplish this. The engine needs to be doing a few revs in order to get gene charging, say equivalent to 30 MPH highway speed; it will not show a charge at idle.
These generators do not have any voltage/current control. You manually set the third brush to regulate the charge rate. The cutout on top is merely a one way switch; when generator stops turning, the cutout [switch] opens to prevent current running back from battery, which would discharge [flatten] the battery.