I might be having the same issue with my neptune.  When I put it together, I simply  mounted the engine to the firewall and away I went.  Plane flys great!!! once its off the water..my question is how do you add the up up thrust?  Ive heard adding washers to the motor mount will change the T/L.   Maybe this is why mine takes forever to get into the are.  It gets up onto step quick, but just gets pinned there for about 100 feet