1981 cb750 not charging right

[Kevin]

So I have an 81 cb750c that I am having battery issues with, I know that the rotors arent the greatest on them, I tested the rotor and it didnt seem quite right, but it wasnt dead completely, and if I put the multimeter on the battery when the bike is on and rev it up a bit the voltage does go up, but only to about 13.4 or so, so my question is, would that come from a bad rotor, or a bad regulator/rectifier? I just want some other opinions before I go out and spend either $110 on a rotor or $90 on a rectifier/regulator
Thanks a ton
-Kevin

[sgtslag]

Does the rotor come up with acceptable resistance readings, according to the FSM?  The FSM is available for download from the Home Page of this site.  The FSM also gives test methods for the Rectifier/Regulator circuits, as well.

It sounds like your rotor is putting out AC, but maybe not enough voltage, since you only get 13.4 Volts on the battery posts, when revving the engine (above 2,000 RPM, I trust?...).  If the alternator is not pumping out enough amperage/voltage, then the battery will drain, and die.  I assume this is the problem?

Check the resistance readings for the alternator, and the Rectifier/Regulator, against the FSM, and you should know which direction to go.  Best of luck!  Cheers!