These motors are the model C 350's I had the original model A, the model A and B had flywheel issues apparently the model C's still do, but there is a flywheel counter for service hours at a certain RPM. The “C” designation denotes a third generation of the motor with a number of incremental changes Yamaha has incorporated in recent years, including improved gearcase lubricity and corrosion resistance, reduced weight, and the extension of the standard warranty to five years.
from the website:
The F350C is fitted with an ECU (Engine Control Unit) that monitors time of use in specific RPM bands and then gives the owner an alert when regularly-scheduled service is due. The ECU specifically clocks the time the motor spends between 3500 and 4200 RPM to address an issue with the flywheel design of the F350. A few years after the F350 was introduced it was discovered that a “harmonic event” takes place within the engine between 3500 and 4200 RPM that over time could cause the flywheel to fracture, leading to major engine damage. With no apparent fix for this issue short of redesigning the engine, Yamaha has a policy of simply replacing the flywheel after the engine has run for 80 hours within that 3500-to-4200 window, and set up the new ECU to count those hours. Note that this interval is not for 80 total hours of operation, rather only the hours in that RPM range, which is likely below the best cruising range for most boats. The performance of each boat will determine how much time the motor spends in this state of harmonic dissonance. This ECU has been retrofitted to earlier F350 models at no charge for a few years, and Yamaha also picks up the cost of replacing the flywheel. Perhaps to boost consumer confidence, Yamaha has also extended the base warranty of the F350C from three years to five years, with an additional two years of coverage available at extra cost.