1981 Johnson 50hp overheating issue.

Kurtis514

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May 26, 2019
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13
Hi guys! Hoping someone can point me in the right direction here cause I’m at getting a bit frustrated . I got a 1981 Johnson 50hp I’ve taken over from my old man as he’s just left the boat in the bush for 5 years and it’s a damn shame. I took it out on the water for the first time and it ran alright for a bit but overheated near end of run. Took home.. replaced water pump and thermostat was stuck open so replaced it too. Second time out and it ran ok at first but overheated again. Ripped it right down (removed power head and lower unit, opened water passage), found it had a blown head gasket. Replaced all gasket and cleaned out everything. Put a straight edge on head and has very minor warp but nothing I was concerned about. Anyways got it all back together (torqued to spec), put the ears on and after about 5 minutes she overheated again! Thermostat confirmed working. There is no pressure release valve, it’s been silicones over. Any ideas gentlemen? Thanks and sorry for the long post.
 

Kurtis514

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Joined
May 26, 2019
Messages
13
I should add I was getting 125 psi with the blown gasket. Now getting 120 on both cylinders. Is there anything else that could cause this other then cylinder exhaust getting into the cooling system? I’ve read about deflectors swelling or breaking but I don’t think they’re are in this motor. I didn’t see anything when I had it opened up anyways. Strong tell tale stream also. Thanks for any help that could be provided!
 
Joined
Jan 17, 2019
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946
Well the pressure relief valve isn't there for good luck! It's critical it stays sealed at low speeds so water gets to the top of the head. Also even more critical it opens and allows water to flow into the head at high speed. it's overheating because the poor thermostat is overloaded, and isn't enough volume to cool it sufficiently at high speed. Normally, with the valve working, the t stat gets completely bypassed! Get a new spring, grommet and valve. They are NLA, I think, but can find some on ebay now and then. I have a good used set if you need it.
 
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also, check the plugs to be sure water isn't leaking into the cylinders. Will be steam-clean if it is ingesting. what gasket did you use? I ask because the newer ones that are tacky use no sealant, but older ones and off brands might need it if its not tacky.
 

Kurtis514

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Joined
May 26, 2019
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13
Thanks for the reply! My father Used this boat and motor for quite awhile (20 years) and had never had the motor opened up in that time and it’s ran good. So that valve was never there this whole time. If that is the case it’s strange that it’s causing problems now and not before. I have it all apart again. So if I remove the silicone and put it back together, then I guess that would confirm it wouldn’t it? Thanks again for the reply!
 

Kurtis514

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May 26, 2019
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I attached a picture of the gasket. It has do not use sealant right on it haha. But I think I might try some. What type of sealant would you recommend? Thanks.
 

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Joined
Jan 17, 2019
Messages
946
no worries glad to help. No, that gasket is good to go without sealer. Back to the pressure relief valve: if it was really and truly sealed up, it would've been overheating at high speed. You absolutely need to clean it up, put a new grommet, valve and spring in there. no other way around it. make sure the t stat is really working. Once you do that properly, check the temp at the top of the head. should be 125-155 degrees, and not a penny more at idle. if it's over that, you've got a blockage from corrosion, or a leaking grommet were the water tube goes into the powerhead, or the exhaust seal *rectangle shape, was rolled/pinched when you installed the lower unit, which prevents the "wall of water" from cooling the exhaust in the midsection.
 

Kurtis514

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Joined
May 26, 2019
Messages
13
Awesome thanks!! I actually have an old 55 hp I was going to fix up and it has the pressure relief valve in it so I’ll put it in. Hopefully it solves the problem! Be putting it all together tomorrow and hopefully take her for a run on the lake tuesday! Thanks again! Much appreciated!
 
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