Most outboard motors will run fine for years with little or no attention. If you notice that your outboard motor is running at a slow speed or idle problem then you should consider getting a major tune which includes:
Removing cylinder heads to de-carbon pistons and heads
Remove and rebuild carburetors
Replace thermostat(s) and water pump
Re-torque cover bolts, etc.
Check and/or reset all adjustments
Change and/or re-lube as needed
Make sure that you are using a good boat motor oil like the Yamaha 2s oil which helps to protect the life of the motor.