Could be the gen is charging adequately but the ammeter does not show much. That is how mine is on the 52 Merc. Only time I see it much above the centre line on the ammeter is after a hard start with lots of cranking when the carburetor had drained itself. Otherwise the needle is pretty stable on the centre mark of the ammeter. Turning on the lights has a slight effect towards the discharge side.
At idle without lights in I sit at 6.1v once it revs up the most I've seen is 6.25v. lights on the needle shoots half way to "d".