Delco 35A generator on a C172 with a TCM O-300D. On start up and take off, system voltage is at 14V. As flight continues, voltage slowly drops. After 15 min. it's 13.1V. After 30 min. it's 12.6V. After 45 min. it's 12.1V. After 60 min. it's 11.9V.
Ground adjustment after flying (still warm) requires 2 turns of regulator voltage screw to bring back to 14V. The next flight a week later, the voltage is now at 17V. Adjusting the screw again back to 14V brings back the original scenario. Voltage measured with external voltmeter to bus bar.
Have not trouble shot other than checking that the wiring is tight. Cleaned a few terminals. Grounding is good. Generator commutator and brushes look good. Battery is 4 yrs old, but looses charge after 6 weeks of sitting.
Whadda ya think ???