The plane should get off the water with a 40 LA. If not --- is the engine propped correclty? What rpm with what prop? Is the plane balanced according to the plans - 16" ahead of the TE of the wing - not of the ailerons? If you moved the tank forward, why in the world did you put the additional weight of servos back in the rear? That defeated the entire purpose of moving the tank forward. In addition to not knowing what your balance point is (CG) I would think that putting a 40LA and its tank that far apart - and therefore the tank well below the engine (much less the carb) - has got to have a significant negative effect on the engine operatiopn.