Governor should have no affect - provided it isn't surging or rev spiking I would negate it.
I still reckon its running rich.
My diagnostic process would be:
What spark plug is in it? bp5es or 6es? If 6es, swap to the hotter 5es for cleaner burn
Confirm with a honda dealer (with the model number) if a snorkel requires a smaller main jet to be fitted (I believe it technically should)
Inspect bore/head with an endoscope and confirm carbonisation levels.
In any case, the first thing I would do is start running it on 98ron with a Polyether Amine based injector cleaner. This ingredient will help most with decarbing. Most cleaners don't have it, Caltex techron, stp complete fuel system cleaner (silver bottle) or rislone High-Performance Injector Cleaner all have it. As does Yamaha ring free (go to a boat dealer).
PEA is the best decarb fluid around
