Impeller
If it is a Yamaha I would say every 5-10 years. Impellers actually get worse sitting than they do running. When sitting dry the rubber can adhere to the outer housing and when you start it you will pull chunks off. I have serviced many Yamaha 4 stroke outboards and if the pressure coming out of the little squirting hole in the back is strong you are probably good. I would also say if you store it for long run some propylene glycol through a set of muffs before you put it away and that will lubricate it from sticking.
-Happy Boating.
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