You are using a multimeter to confirm 60Hz? Have you looked at the manuals for default settings for the CIM. Check out our free manuals here: https://greenmountaingenerators.com/manuals-and-support/mep806a-manuals/

There might be default configuration settings to be programmed. Does the readout change if you adjust throttle? Just need to confirm if this is static or moving with the RPM.