Have you checked carefully all the way around the crankcase joints for leaks? Note that this engine uses sealant, not a gasket.
I'm pretty sure you will be aware of this, but the jets (main and idle) and the emulsion tube are prone to collecting hard gum, which will not disappear just with a quick spray with carb cleaner. I cleaned a Honda carburetor last week that had its main jet, its idle jet, and 14 small holes in its emulsion tube, clogged completely, and unable to pass carb cleaner. I had to put jet drills through all of them, then follow up with carb cleaner. I had another (same model) Honda a few months ago that ran just slightly lean (not bad enough to cause hunting) and I found its main jet was down from 0.55 mm to 0.45 mm due to gum. I was unable to shift the gum with oxy torch tip cleaners, but after a good going over with the tip cleaners, a blast of carb cleaner fixed it completely.
I tend to remember the jet and emulsion tube hole sizes now, and am not satisfied until I get a jet drill of exactly that diameter through each hole.
