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How can you be trouble shooting and working on a motor without having the applicable service manual on hand?
Appears to me you are doing maintenance for compensation. Why are you depending upon the largesse of the Iboats community to help you in a commercial situation?
Thermostats are used to help the block to warm up faster and to ensure that the block can come up to operating temperature in cold water. They are purely mechanical and are not connected in any way with the thermoswitches or the thermosensor.
The thermoswitches close if either is over heated...
The motor would have left Yamaha with a round decal on the block. Usually in the vicinity of the starter motor. Has some numbers on it. What are they?
Look at the CDI assembly. What numbers/letters are on it?
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.
