Take a look at the Yamaha break in procedure. Then take a look at the Mercury Marine break in procedure.
Then you might have some idea as to why Yamaha's suffer from making oil at a higher rate than do Mercury's.
Yamaha calls it Primestart. An automatic fuel enrichment system. Adds extra gasoline when the motor is cold.
Not uncommon for a 21 year old motor to be seriously in need of a thorough cleaning.
It is a stretch, but what with some carburetors having miniscule orifices and passageways perhaps the slightly higher viscosity gas/oil mix might be hindering the flow of fuel to the motor.
If 75/1 works better than 50/1 one could presume that the Yamaha recommended 100/1 gas/oil ratio might be better. Cost little to nothing to give it a go.
There are a number of Yams designed to run on 100/1 premix and have no less of a life than those designed to run on 50/1, or on auto-lube which in some cases has a ratio of 200/1.
Where in the world are you? I have no idea what RNLI means.
When you say "I had the neutral switch bypassed on the Evinrude" I am presuming that you mean the control box neutral safety switch was bypassed. Not a particularly smart idea but so be it. To each his own.
There is no wire to bypass in a Yamaha motor. If battery power is provided to the motor...
I remember the days of 16/1. Folks were scared to go to 25/1 and added extra oil.
Then they went from 25/1 to 50/1. Folks were scared to go to 50/1 and added extra oil.
Now they go to 100/1 and folks are scared to go to 100/1. Adding extra oil.
It is not abnormal for there to be a period of time before water starts coming out of the tell tale. A minute's time would not concern me.
Having said that, when was the last time the water pump was replaced?
