I was talking with one of my clients this past weekend and he has had some problems with his big Honda four stroke. It seems there are two small exhaust ports on the back of the engine just above the waterline and when water gets in them it screws up the engine's oxygen sensor. He told me he could not trim it all the way down because of this...don't know if this is the problem you are having or not but thought I would throw it out there for you.
Dave