OK, if it runs OK if you slowly advance to full throttle then your idle mixture is too rich. I know, hard to believe but it's true. The engine is loading up on you at idle. Lean the idle 1/8 turn increments. It's the small screw INSIDE the carb rotating barrel. Either be EXTREMELY careful or preferable for a beginner, stop the engine while adjusting the idle mixture. Too close to the prop for safety.

After a couple of idle adjustments re-tweak the high-speed mixture too. Lean until it peaks and then turn back CCW a few clicks to the rich side for consistant running and to protect the engine. Once the idle is set it will probably never need touching EVER again unless you make a drastic environmental change. OS tends to ship their engines a bit on the rich side as a precaution.