[dhgeyer]

At the risk of asking the obvious, have you run a couple of tankfuls of fuel treated with Chevron Techron Fuel System Cleaner through it? Or a similar product? Some people swear by Sea Foam. Both available at just about any auto parts store. Be careful to use the right proportion. The directions will say something like "Treats up to 20 gallons", so you don't dump the whole bottle in your little tank.

Carbs get gunked up by the lousy fuel we have to run these days. They're not as simple as they used to be, and if anything's clogged or not moving freely you get a lousy running engine.

The float valve (or the float itself) got stuck in my carb a week or so after I got the bike. I ran a couple of tankfuls of Techron through it and it cured the problem. Carb could use a good cleaning, I'm sure, but it's running fine now, so I'm not going to mess with it till it's down for the Winter anyway.

My daughter's Ninja 250 sounded and acted like your recording after sitting for a while a couple of years ago. Ran some Techron through it and it was fine. Stuff works pretty well.

I run a tankful of treated fuel through my bikes every 5 to 10 tankfuls as a routine practice now, more often if they sit for a while. Seems to prevent problems before they occur.