The basic problem is that the airplanes are nose heavy. The more forward the cg, the more stable in pitch so the more they will pitch up with addition of power. As the cg is moved rearward they will become more "groovy" tending to go where they are pointed without pitching up. Move the cg a little at a time, and when it flys like you want it stop. Too far rearward and it can become unstable and hard to control. A little downthrust will help control the pitch up tendency if you leave the cg a little forward.