RE: Goldberg 67" Yak 54 ARF
Saito 125 with a 15x4 is under proped. A 125 is lite weight, but does not have the umph for this size plane. Try a 17 x 4 apc and 30 heli fuel and see what she does. I use a saito 180 and a 17 and 18 inch and have plenty of umph and prop outside of the cowl.