My experience is that Victron inverters can get grumpy with dirty power from an open frame generator. I had to deploy software settings Victron describes in their generator FAQs which resulted in slower charging and potentially passing through power that's not ideal for sensitive electronics.
My #1 reason for using the EG4 100 amp battery charger - I refuse to use their stupid marketing-bred brand name - is to only supply the house with clean Victron inverter AC and never pass through potentially questionable generator AC. A discreet battery charger avoids the risks that Victron describes below, e.g. when a substandard backup-backup generator is used.
Can one use a sub-standard generator and adjust the Multiplus II settings to charge the batteries and pass-through cruddy power to home appliances? Yes, but why do that when a $450 battery charger totally eliminates the possibility of ever needing to do that.
From Victron:
www.victronenergy.com
Software Settings
The settings which you should attempt first are 'Disable UPS mode' and 'Enable Dynamic Current limiting'. These compromise features, but maintain the quality of the power supply.
If the generator will still not synchronise or disconnects, try turning on the setting for Weak AC. This will compromise efficiency, but allow for a wider variation in wave form distortion to be accepted.
Lastly, adjusting the settings for Voltage and Frequency windows. These are higher risk to your appliances, they should only be used cautiously when it is confirmed that the loads will tolerate the out of specification power supply. This should be done by a competent technician with the aid of a multimeter and only opened so far as to allow for safe functionality.
Electrical current will go through the MultiPlus only if it meets the programmed minimal quality requirements; in that way the MultiPlus protects the electrical supply of delicate equipment.
Warning: changing these settings will allow to feed-through current that may be of insufficient quality for the electrical requirements of your equipment.
For information on VEConfigure and how to get started, please see this VE Configure Software page, or contact your local Victron dealer.