This seems to be a very common issue with these engines to have a rough spot in between 2000-3000 RPM.

Mine ran rough below 3000 RPM when I got the plane. I rebuilt and sync'd the carbs and replaced the throttle cables and now it idles absolutely perfect but I do still have a bit of a rough spot as well right around 2500 RPM. It does not seem to be affected by a mag check either.

These carbs are a 3-phase carb where they have a circuit for idle, midrange, and top end. The air adjustment screw is the mixture for the idle, the idle jet is for the midrange, and the main jet is for the top end.

At idle and top end I seem to be dialed in perfectly but below 3000 when the midrange jet kicks in I get a bit of a rough spot as well. I have read about people with similar issues changing the idle jet from a #35 to a #45 and having good luck with that. That #35 jet has a very tiny orifice and is prone to getting plugged up by very small debris as well.

It hasn't bothered me enough to do anything about it yet but I did want to try switching out those jets to see how it works. You can get the jets from a motorcycle parts supplier if you don't want to pay the aircraft part surcharge as many motorcycles used Bing carbs and the jets will interchange.