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my fishing partners boat has lost around 4 mph on top end speed.he says the engine(200 hp efi merc) still turns the proper rpms.i know nothing about outboard engines but he thinks he is loosing compression.is it possible to loose compression and still turn the proper rpms.thanks for any help
You can pickup a compression gauge for a few bucks there is a lot of "fourms" about how to do it. Just do a google type search for outboard compression check. Also it might be carbon build up, do you know if runs any type of fuel cleaner?
air temperature can have an effect on how your outboard engine runs.my dad says his 150hp will run 3mph faster in 50 degree temp than in 90 degree temp.i dont know a lot about the compression and rpms but you would think if you are loosing compression you could not turn the proper rpms.
First place to check is the prop. If it's not perfect you will lose speed. Load change will also change your set-up...