Rough = not balanced well.
Ask them.
vario Benzin Acrobatic heli
Phoned them today concerning the engine and they say the heli is designed around the engine and has a modified carburettor, shielding ect and are well balanced, along with a tuned silencer. .......we shall see. I asked about vibration and the answer was at 24 cc its fine but whith the bigger bores it may become a problem. .......their words not mine .