Hi Occidio. IF your engine has been running good until now, I'd check these things before changing needle settings.

Has the idle stop moved? Spring tension strong enough to keep the gap consistent?

Replace your plug unless it's new. Sometimes new plugs are bad from the factory so check it anyway.

Make certain your fuel is fresh. Make sure your fuel system isn't clogged. Fuel line is cheap and easy to replace and also eliminates a possible air leak.

Next easiest is the carb. Make sure it's well seated against the crankcase. I'd go ahead and clean it if you haven't. Just keep track of needle settings.

Check the head and back-plate screws to make sure they are tight.

If it's still out of tune, refer to Ron Paris' "engine tuning bible" at the top of this forum. He was The Man and his writing is among the best recourses ever.

Good luck