Compression tests are always good. First rule out major mechanical before messing with anything else.
Originally Posted by RIB-Teccie
Priming fluid will ruin an engine as it washes the oil off the bore and over time causes engine wear and loss of compression.
If you are referring to "starting fluid" it may cause damage over time, but a use once or twice as a diagnostic will not do any harm, unless you over use it. A single shot into each carb is all it takes.
Extra Strength Starting Fluid for the fastest starts
Starts engines quickly and reduces drain on batteries
Compatible with gasoline or diesel engines
Contains upper cylinder lubricant
It is a quick way to see if an engine is getting enough fuel. Then once diagnosed it can be properly repaired.
Does this Yamaha have "Prime Start". Do you have the owner manual? If not I recommend buying one.
Does the motor have one of these levers?
What is the year of the motor?
I used to have a Yamaha 90HP years ago. It was a fantastic engine that only failed me once when the starter bolt fell down. Put the bolt back in and it fired right up. Other than an occasional impeller, lower unit oil change, and spark plugs it never needed a single repair over 100's of hours of use.