You may have missed cleaning the jet on top of the carb. You do not need to take the carb apart or remove the carb from the engine. Remove the screw that covers it and clean that out. I use a fine brush wire and a few good shots of wd40 or equiv. oil.

I have the exact same problem, exact same engine. Unit only 2 months old. I was told it may be the idle circuit. Love the unit otherwise.

'under a load it runs great (full power).'

Then as far as actually using the mower, you don't have a problem. Try running some injector cleaner through it.

Me and a friend both had this problem with our mower, they would only run on high speed and would pulse or surge at idle. Tried cleaning the carb but that didn't work, tried replacing all the gaskets, no improvement. Bought a new carb from Plano Power Equipment for a whopping $16, along with the governor spring and rod, spent about $31 total. Installed the new carb and it purrs like a kitten, idles, runs on any speed, just like new. I did also move the spring on the lower right that attaches to the governor lever from one hole to another. Seems the carb and associated linkage is very touchy, that's why I replaced the other parts but the carb may have had a clogged jet and I just didn't feel like messing with it so replacing it did the trick for me.

Not sure why the Honda card is so tempermental. Seems every Honda Mower suffers from this.