Probably because these engines sometimes come with retarded ignition timing. The Saito FG 57cc should spin a 22 X 10 propeller with good authority and good throttle response. If not...
try advancing the ingition timing slightly and see if the symptoms improve.
Also, the symptoms described might indicate a rich fuel condition caused by the lower needle needing to be closed about an 1/16th turn.
Please private message me your email and I will send you some very informative tips. I already have them written up to help fellow modelers.