Fuel or carb clean

Get a can of Berryman B12 Chemtool. Add a few oz to the next fill up. If the engine smooths out under load, it is lean surge and was set that way at the factory to satisfy emissions. If the mower has a fuel shut off, use it every time you use the mower. If it doesnt have a shut-off ,put one in. I also add a cap of MMO when I fill up the tank. I have had a lot less carb problems since I started doing this about 5 yrs ago. I don't really know if the MMO does any good. Thats the mystery part. But in the fine print of my 5kw genset ops manual, it advises to run the carb dry after every use. The smell is the marvel. The oil is something like kerosene

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Here is a link that might be useful: His Mower

The word is 'hunts', like its looking for the right RPM but isn't finding it. I don't know, possibly an air leak or a damaged governor spring. Grab that governor arm and race the engine a few times to see if it will smooth out.
Tell you what, that B12 is very strong - don't use but a quick splash in your mower tank, 1/2 oz at most. The Marvel Mystery Oil is a great maintenance product and mowers love it, 4-cycle or 2-cycle. It would be difficult to use too much. You could easily use 2 to 4 oz a gallon if you wanted and the engine would be happy. As to what immediate effect it might have on your current condition, I wouldn't count on a cure. I use it all the time along with Sta-Bil fuel stabilizer and I don't have many fuel system problems. Actually, none.

At least the Chem Tool people label what is in their product and it is strong solvents like xylene. MMO doesnt list their ingredients I noticed.

You can Google it. The only 'active' ingredient is a 'Stoddard' solvent, probably about as strong as kerosene. Add color, a highly refined mineral oil and the pleasant scent. It's just a solvent oil, and could make your own, but MMO has been around for almost a century now and the actual formula is still a family secret. In all those years it has helped millions of engines run smooth and clean. I had a '76 GMC van with a 350 that was lightly used, and would develop sticky valves, giving it a little 'burble' in the idle. A dose of MMO always took care of it. I use it in OPE fuel and as-needed in my vehicles in the oil. I've noticed these tight-clearance DOHC engines of today tend to give some valve train noise on start-up sometimes or a lumpy idle at anytime - MMO always smooths them out, even the 4-cyl.

My HRX does this as well. It used to piss me off but I ignore it now since it runs fine at full throttle.

LOL Same here with my old HR215. Starts easily and cuts like a champ so I don't worry about it. Maybe this winter I'll get a round toit and build a workbench in the shed so I can give the old girl a freshening up.

If your type is one where you start the engine and then engage the blade, I wouldn't think it strange that it hunts, because the blade helps add weight to the crankshaft; another flywheel, if you will.

I worked on one yesterday that did the same thing, but when the blade was engaged, it worked perfectly.

It's no different than the Toros that use the same system. They do it too and so do snowblowers.

I didn't think about the blade brake angle, that probably explains it.