25 johnson model 25R75B overheating

[bfryer]

Just aquired a 1975 Johnson short shaft. Ran it first in a barrel. Would pump water with water exiting the relief port in the mid section then would quit and then start again. I went ahead and changed the water pump impeller. Started the motor and water would exit the relief port in mid section then quit. Motor then seems to get hot enough I can't leave my hand on it. After a moment the water starts coming out the relief port again and the motor cools down. Then the sequence repeats itself again. Next I took out the thermostat and ran the motor. I am getting the same results. Water is present at the relief port then stops and motor starts to heat up. When water at relief port does start again it comes out in a gusher. any suggestions?

 

[OBJ]

Re: 25 johnson model 25R75B overheating

bfryer...let me ask a dump question, do you have enough water in the barrel? OB water pumps will not draft water. They need to be covered with water to work. Make sure the water level is well above the line where the lower unit and midsection mate.<br /><br />OBJim

 

[Jack Shellac]

Re: 25 johnson model 25R75B overheating

Did you use a new impeller pin? Sometimes an old pin is just worn enough to let the impeller slip on the shaft and then catch it again giving the symptoms noted.

 

[sailor_turned_stinkpotter]

Re: 25 johnson model 25R75B overheating

Could be a small fault in head gasket that opens up when motor gets hot and then the combustion gasses from cylinder blow into water jacket and force water from pump back down the leg....sort of like a mexican standoff...water wants to come up and the leaking exhaust gas wants to go down and nothing goes anywhere, untill the motor cools and then process repeats.

 

[sailor_turned_stinkpotter]

Re: 25 johnson model 25R75B overheating

Oops, I forgot to tell you how you could check it. Do a compression test when the motor is cold, then again when the motor is hot, if you notice a bigger difference on one of the cylinders is might pay to pull off the cylinder head and have a looksy.

 

[bfryer]

Re: 25 johnson model 25R75B overheating

Thanks guys for the advice. I will first check the water pump again and go from there. The barrell was full up above the cav plate on the lower unit. Initial compression checks indicate 130 lbs vs 131 lbs taken after the motor had been run. I will check the compression before and after I crank the motor. Let you know next week. Probably monday. thanks again

 

[OBJ]

Re: 25 johnson model 25R75B overheating

bfryer....just a double check here....the pump sits above the cav plate. This is the "fin" right above the prop. Look for the mating line between the lower unit and midsection and make sure the water level is well above it.<br /><br />OBJ

 

[bfryer]

Re: 25 johnson model 25R75B overheating

ok will try more water tomorrow. let you know ben.

 

[bfryer]

Re: 25 johnson model 25R75B overheating

come to think of you may be onto something

 

[crazybear-uk]

Re: 25 johnson model 25R75B overheating

Hi <br />i had an old evinrude 40hp short shaft of the same vintage. had identical symptons to yours unless i could get the water level above the bottom of the transom bracket it would not pump correctly and did what yours is doing. i replaced every part of the pump system and it was still the same. out on the water she was fine and would run perfectly all day with no signs of overheating. i ran the engine for many years and it never let me down it was just a pain to flush out at the end of the day. if you have a back up engine put her on the water and check her out may be the same<br />regards<br />mark

 

[G DANE]

Re: 25 johnson model 25R75B overheating

Hi bfryer - Carzybear<br /><br />Ditto - I done exactly the same first changed impeller, then complete pump then after $$ put more water in barrel and viola !!

 

[bfryer]

Re: 25 johnson model 25R75B overheating

more water in the barrel did the trick. I thought water above the cavitation plate would do it but I guess not. Learned something today. thanks for the valuable information.