There's so much going on there. Generally, the charging coil output should be held in check (14.5v max) by a regulator. It should not allow voltage to increase beyond that, no matter the rpms. The regulator might be integral to the rectifier, but could be a separate component - depending on design. Not being familiar the the BF50's design, can't be of much more help.
Note that a low (or bad) battery may hold charge coil output at a much lower value. Note also, all that may have nothing to do with your tach issue. The tach irregularity on idle could be the regulator turning the charge coil on and off while doing it's job trying to keep a constant voltage. You could try a known fully charged battery (13.5v) to see if that changes anything.