I posted this in Antique Engine section and didn't get a response - though t it might be in the wrong place so added here.

I have a 1975 Mercury 200. I have just got it back from a full check and service including a new impeller. We have test run the engine up in a tank and it's very sweet, starts well, revs freely and happy at full throttle to idle. It has obviously never been run in salt water. My question is how often or in other words how long can it be left before it needs another run up to keep the motor and particularly the impeller in good order. Am I correct in my understanding that impellers dry out if left for too long and not run. As yet the engine has only been run up in the tank after the rebuild.

Thanks.

The impeller will not like standing for a long time. You could run it every 4 - 6 months BUT, if left without use, before proper (i.e. on a boat) running you should fit a new impeller. You can turn the motor over by hand regularly which will help but the impellers don't like standing & get a set on the blades. For what is involved I wouldn't chance it. Some people remove the lower unit & take the impeller out when not being used. I change my impellers every two years regardless, & they get used from March until late autumn normally.