onan gen problem

i have a 1986 63 cpmy with two 20kw onans. one of them is tripping the overspeed breaker after i switch off the laod. i noticed the variance of hz and volts when laoded vs unloaded is much greater than the other gen. loaded (50a) it runs at 58hz and 225v. unloaded it runs at 65+ hz and 260v. while the other one runs at 62hz and 240v loaded (same load) and 64hz and 245v. i would appreciate any input on what the problem might be. thanks in advance

Sounds like you need governor service. Frequency is directly related to RPM.
2 pole units turn at 3600 ( 1 cyl toy construction gens )( they scream! ) and 4 pole units turn at 1800 to provide EXACTLY 60Hz. At 62Hz your volts are slightly higher also. Overspeed trips are usually 1-5% of running speed. ws