1972 50 Johnson overheating

[lurch67]

Good Morning Boaters, I just got a 1972 Johnson 50 hp. I got is started and after about 5 minutes of running on muffs the alarm went off. I replaced the water pump the same way that it was installed. Started it again, the alarm went off. I pulled the lower unit again and realized I put the impeller in backwards orientation. I flipped it around and reinstalled it and tried again. Now I got some water coming out of the lower unit pee hole. but after a while the water quit flowing and the alarm went off. I have now replaced the thermostat and tried again. No water came out. Alarm went off. I have pulled the lower unit again, pulled the thermostat, I now wanted to test for blockages so I ran water up the brass tube and saw it come out the thermostat hole and the pressure relief hole so I know there are no blockages there. I also have ordered an new impeller and a new housing. If this overheats again, I don't know what else to do. How does the head get cooled? Does the water pressure push the spring back and circulate in the head until the thermostat opens? Where does the water go from there??

 

[lurch67]

Well, Update here. Always run these old motors In a barrel or test it in the lake. It is running and cooling properly now. My new problem is that when I slam the throttle down. It doesn't take off. it eventually get up to a plane and tops out at 25 MPH on the lake. I am doing a carb job next. Is there a link anywhere to a FRM for this motor? Moral of the story. Always try it in the lake before you tear it all apart. Muffs might not always work.

 

[MegalopsAtlanticus]

No doubt!

Sometimes those older OMC's need the lower unit submerged pretty deep to get the water pumping right ('prime the pump', I guess).

I had an '84 Johnson 30hp that wouldn't pump in a trash can... But would pump perfectly when on the lake. Pretty weird phenomenon, but I got used to it.