Genset stops charging

I have a relatively new Westerbeke 4kw genset. It gets little use during most of the year then is depended upon for our annual vacation.

This year, I have had mixed results. sometimes it works properly. At other times, the voltage it puts out is too low for my Xantrex MS2000 charger. It won't accept the incoming voltage, so no charging occurs.
I called my mechanic, who did the original installation, and he said it likely has a defective voltage regulator. He was concerned to know the Hz it was putting out, as he was concerned that too low Hz would damage some or all of the equipment connected to it.
61 to 63Hz was consistently made when generating.
95 to 105 v was insufficient to get the Xantrex to turn on.
when the generator strayed above 105 v, the Xantrex would turn everything on, and as soon as that happened the voltage would go up to 125 and stay there for a while. After charging at 90 amps, declining slowly to 20 amps, everything worked spendidly. Occasionally, it would continue to work perfectly down into the low teens v, but more frequently, once below a charge rate of 20 v or so, the voltage would drop and the Xantrex would no longer accept the incoming voltage and would shut down.

Is this performance symptomatic of a lazy or defective voltage regulator in the Genset? If not, what is going on?

Despite all of this, we were able to enjoy 3 weeks on the hook between shore power hits. Prawning was great, weather was as well.
Pendrell Sound was as warm for swimming as I have ever seen it.

For those who have asked where the avatar picture was taken, we spent several days there this year and as we were earlier than usual, had it all to ourselves 5 nights, shared a few more, but never a crowd.

Barring a very bad connection somewhere the voltage regulator should be maintaining 120 v +\- 5%. The spec is listed in the generator manual. Typically higher voltage at low loads.
One easy way to monitor the generator is with a “Kill-A-Watt” plug in monitor.
WB have a 5 year warranty.

It does sound like the generator.

I don't think that thing even has a voltage regulator. It has a winding and capacitor on the stator that induces a field in the rotor, then diodes in the rotor do something that sets up a fairly stable output.

If capacitor is weak, output volts will be low. And you can up the capacitor a bit to increase voltage. So if capacitor tests well at the labeled say 30MFd, you can try a 32.

Test the capacitor.

Hz looks good, light load should be around 61-62.

This is just passing on dock talk, and I may be remembering it wrong, but here goes. Try putting a modest load on the generator, like a 100W light bulb, and see if that brings up the voltage.

WAG - what load does the generator have to support? Anything over 30 amps or so and the voltage will drop.

Pendrell was too crowded when we were there, didn’t stay but we have had way less boats than usual this summer, only averaging 12 in Grace and Roscoe. Octopus was very empty. One other boat with us now (in a secret location!!).