Slowly narrowing the bugs in the boat, now I'm concentrating on the engines performance while in water.
Right now I need to clean (or rebuild) the carbs and check timing cause it's running way rich, to the point of fouling the plugs.
Photo of fuel consumption at 31mph and rpm. *:-[

New plugs with 30 minutes run time on them (after being wiped off with a wrag).

It will quickly do 41mph at 5000rpm (and draw 22.5gph).
I was expecting about 8gph at 30mph.
We'll see when done with the adjustments if it improves. :-/
By the way this wasn't at best trim, speed, etc, etc.
We were just trying the motor's reaction to various circumstances. I'm sure the numbers would have been a little better by picking the best operating range (which was what I was trying to do till it started fouling the plugs).