Were the instructions written using a 4 stroke? If you move the Cg up (toward the nose) you're going to make it more tail heavy. If you're at 4 inches and the tail is still heavy, you might have to add weight or put your radio and battery in front of the cg. my Ultra Sport and Ultra Bipe were the same way They call for a .40 - .60 2 stroke or .70 4 stroke. The .70 4 stroke is about the same weight as a .60 2 stroke.