Quite often an engine will struggle the first time after start as you come up to speed. This CAN be normal. The test is to get it up to full speed, return to idle, remove the plug battery and see if you can accelerate the engine smoothly to max. You should be able to just go from idle to max without hesitation or sputtering. If it quits or sputters you will need to lean the idle mixture carefully in 1/8 turn increments until you are satisfied. OS usually ships these engines too rich. Better that than too lean and burn them up is their motto. Be very careful adjusting the idle mixture, I recommend stopping the engine to do this. Sharp prop. [:@]
If flying with a club or experienced fliers, they should be able to help with this.