1975 Johnson 50hp Overheating

[timmeraw]

I need some advice please. Just picked this motor up. Checked lower unit(milky btw and I replaced lube...probably need seals) and checked compression (good on both cyl). Next I fired her up with muffs on.....a little rough cold idle etc but starts right up. First thing I noticed is that this is one of those foolish models with no tell-tale. Figured there should be some mist at least coming out of the exhaust holes in the back of the upper leg but none. After running for a couple of minutes I was noticing that the head was getting hot to the touch and then bang the alarm went off and I shut her down.

Figured next step would be to replace impellar. Ordered one overnighted to nearby Napa and tore it down this morning to take old one with me for reference. First thing I notice is that impeller looked fine but I had to get the new one anyways so I replaced it. Fired back up and same thing...no misting and head getting too hot.

Thoughts? Thermostat or pressure valve???

Now to my question(s).
1) I believe that the thermostat and pressure valve is only accessible by pulling off the cylinder head cover right?(another foolish design imha)
2) I guess that if I replace them I had better also add a tell-tale

 

[timmeraw]

Re: 1975 Johnson 50hp Overheating

I also see that there is a spring valve and grommet valve. What are they and should I also replace them while I am at it?

 

[timmeraw]

Re: 1975 Johnson 50hp Overheating

Update: I was thinking that maybe it wasn't getting enough pressure on muffs so I put it in can with lots of water. Although much quieter , still no mist or water out the exhaust vents and was getting very hot again before I shut it down.

I really didn't want to have to tear the cyl cover off but looks like I am out of options. Sure wish it had a tell-tale.

 

[5150abf]

Re: 1975 Johnson 50hp Overheating

It won't hurt anything to change the Tstat, probably hasn't been done since new anyhow, while you have it apart and the lower off make sure the water pipe to the power head is clear by blowing some water up through it. it should come right out the tstat hole.

Also so the water pump on an outboard isn't really a water pump, it needs to be completely submerged before it will move water, even muffs sometimes don't work.

You are doing well to only run it small amounts to see if it is fixxed.

 

[timmeraw]

Re: 1975 Johnson 50hp Overheating

I pulled the cover and it was bone dry. Removed grungy tstat and fired it up and no water coming up tube. I'll pull lower and check the tube as you suggested and also check the impeller etc. Will replace tstat and gaskets as well.

Thanks,
Tim

 

[timmeraw]

Re: 1975 Johnson 50hp Overheating

Wow...this site is getting too big....less than one day and my thread is already on the 4th page .

Update.... installed new t-stat and I believe things are all good now. Had her running on muffs for about 20-30 mins while I played a bit with idle adjustment(yea I know I need to do it again in water) but it got warm but not too hot to touch(for up to 3 secs or so) and was throwing water out the prop. Still wish it had a tell-tale but I am pretty confident it is fixed. Also check compression after it cooled down a bit and was happy to see 150 and 152.

All good and thanks.

Have a great boating season.
Tim

 

[johnnyhihat]

Re: 1975 Johnson 50hp Overheating

hello tim

im having the same issue on my 1975 50hp. i had a mechanic install a waterpump and rebuild my carbs. i got it back running but the motor is running to hot. so ichange the tstat, gasket, pressure relief and spring and put her back together she still running hot. so me and a friend change the head gasket and she seamed to run cooler and water pouring out the midsection. so i ran it for 30 mins in a barrell didnt overheat. so i took her last night and drove her around for an hour and the motor buzzer went off and the motor shut off on me. so i got back to my house and checked compression and i got 90 now. but when i checked it this morning i did get 160 when everything was cooled. so if you find out what it is can you please post it. i will do the same. thanks jeremy