SERGIO..... Before adding weight, Mount the engine as far back as possible
on the mount... Move onboard equipment towards the back as far as it will
go... If you have a mini servo(throttle), that would help, The battery can be
placed as far back as it is needed with some simple mods to help with getting
the CG as you want with the use of an extension for the connection... But some
times you "may" still need some weight, But that should be the last resort....