For what it's worth, my Generac 5500XL RPMs also sag somewhat as a load is added. It has a Generac engine.
The Generac service manual says that with no AC load on the generator, the governor should be adjusted to give 61.5-63.5 Hz (3690-3810 RPM).
Before I adjusted it, it was about 60.5 Hz (per my Kill-A-Watt) with no load. Adding 3000W of load (a little over half of its capacity) brought it down to about 57 Hz.
I adjusted it to about 63.2 Hz. Applying the same load, the RPM sagged to 60.4 Hz.
I don't have other mechanical-governor generators to compare against (my EU2000i throttle is electronically controlled). But my Tecumseh OHV snowblower RPM also sags as a load is added, even before the throttle opens fully. I replaced the governor spring, and the sag was reduced, but is still present. So mechanical governors are not perfect, they don't seem to necessarily hold the exact unloaded RPM. So it seems reasonable to me to set the RPM a little higher than desired, with no load, if that's what the manual calls for.
This message was modified Dec 30, 2015 by RedOctobyr