a friend of mine had the original Yamaha EF1000 that we used mostly to charge his boat batteries on his house boat if it had been sitting and not used for a long time we had to take apart the carburetor and clean it with a can of carb cleaner then it would run great again.
if the voltage is low the frequency will be running low also.
there is a governor and speed adjustment to adjust to correct it there is a service manual available for it.
it was a great generator and he was very mad when someone stole it.
adjustment page