Potential causes/fixes for surging Honda HRX217?

Good morning all,

I recently picked up a Honda HRX217 (not the hydro transmission one with orange lever) off FB Marketplace a couple weeks ago for $500. The old mower's deck was completely shot, so I needed one quick to keep up with my yard maintenance schedule and the Honda was the one I wanted knowing they don't make them anymore, but they're great mowers, so I dropped the coin.

When the guy dropped it off, I tested everything. Cranked fine. Sounded fine. Self-propel worked fine.

Well, the last two times I've cut my yard, it starts surging when cranked (RRrrrRRRrrrRRRRrr). I was able to get the yards cut, but not after a few times of the issue coming back and then going away, and the coming back.

The problem is, it seems to only do it (to a very noticeable extent) while in "Mulch" mode. I eventually had to switch over to "bag" mode and finish cutting.

Suspects:

He sold it with a full gas tank. Maybe the issue is old fuel, but who knows. I sometimes would go a few months using the same fuel in my gas can to fill up my old mower, so exactly how "old" does it have to be to be considered "old fuel"? Luckily, most of the gas that came in it is gone, so my future fills will all be from gas I got recently.

I also checked the air filter. Not the cleanest, but not completely gunked up or filled with dust. Cleaned that out some with compressed air, but the issue still came back while cutting.

Only other thing I can think of is maybe the carburetor is dirty? Is there a specific fuel cleaner I can use to clear out the thing? I also didn't have specific tools on-hand to clean this myself.

Any other possible solutions where I'm not dealing with this every week? I'm sure whatever it is will be fixable, but it's a bummer I just dropped $500 and it's already causing issues.