If you have not been adjusting the governor, there is less than a 1% chance the governor is at fault.
The problem is still most likely in the carburetor.
A carburetor can be the cleanest thing you've ever seen, and still be no good.
The problem is not what you can see, but what you can not see. There are some very tiny passages inside the carburetor you can not get to. It is these passages that create problems.
The only real way to clean a carb is to remove it, disassemble it, and soak it overnight in a commercial solvent. Then use a spray type cleaner to get all of the solvent out of the passages.
Even with a soaking, sometimes a carb will simply not come clean, and it must be replaced.
This is the single most common issue with small engines - particularly ones that do not get used regularly.
To see if the governor is working, simply start the set. Manually try to speed the engine up with the throttle lever and let it go.
If the engine speed comes back down, the governor is working.
But it may still need to have the sensitivity adjusted.
For a calibrated load - it must be precise. Pretty fair will not cut it.
You must apply a full load to the set.
If your set is 3500W, you must put a 3500W load to it.
It must also be a steady load - motors will not work, as the amperage draw is constantly changing.