This'll also happen if lateral balance has been acheived by adding tip weight. Goosing the throttle will cause the plane to try to yaw laterally towards the weighted wingtip because the mass is farther from the thrust line that the mass it was fitted to counteract. The only real solution there is to scrath build, weigh all the materials, install all the heavy stuff along the centerline (side to side, up and down) THEN the prop wash swirl will be the only thing to contend with
