Motor runs great for when cold but bad when hot.

[jayard78]

I have a 1980 merc 40hp that runs fine the first 20 minutes on the water but after that it starts getting loud and does not seem to go nearly as fast. If I stop and fish for a while, it will run 'ok' the next time I start it and go but not nearly like the first 20 minutes


Any Ideas?

 

[rustyh]

Re: Motor runs great for when cold but bad when hot.

i got an 83 merc 40 and had the same problem check yout impeller and make sure its not damaged i was getting a stream off of mine and all i had was one fin left put on a new impeller and gaskets and ran like a dream also you can check the side of the cowling to see if its getting hot .......where i made my mistake was with muffs i was forcing water through the system so at home everything was ok but as soon as i put her in the river she would over heat after cooling off would run like a top until hot again