It sounds like whatever means you are using to charge the battery while it's running has some issue. 14 volts is pretty good for a charging system at work, 12.3 is just slightly low for a battery that is assumed to be fully charged at rest.
I have been able to start the engine which has a 6 volt starter and it runs just fine with with 12.3 volt battery (admittedly low) and it runs very well.
Currently, I have the battery charger on the battery until I am ready to test again after I remove the brushes from the seemingly already dead generator.
