Rating indicates approximate maximum output. It doesn't tell us what it can produce at idle rpms. That's a problem with the ratings.
Some higher output alts can produce more power at idle rpms, and some can't. That type of alternator should be pretty good at lower rpm.
That's good, but won't help if the available power at 14.x is maxed out.
Since it continues into higher rpms, that would suggest something other than power to the headlight circuit being limited than the alternator's capacity at idle.
Might not hurt to put a meter on while running. If a diode is out, or a winding is broken, capacity to produce power will be down and ripple will increase. The latter would show on a oscilliscope - which is not a common tool these days...
