Hi petergomes,check the c/g with the fuel tank empty, if you balance the plane with fuel in the tank it will become tail heavy as the fuel is used while flying causing the plane to become sensitive in the elevator, most trainers balance on the spar if you don't have prints thats a good balance point<your plane is a lite weight trainer?> move the battery is the best way to change the c/g,I add weight as a last resort,as a beginner have the plane balaced level or the nose a little down,one size taller wheels usually don't weigh that much more.Hope this helps,John.