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1983 Johnson 7.5 - Running hot

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FishyBC

Contributing Member
Hello all,
I have a 1983 Johnson 7.5 short shaft that runs beautiful. I do however think it may have heating issues because when I put a Temp gun on the head while it is running in a tank, it is getting up over 170 deg. I have tried this in gear and in neutral with roughly the same results.
I don't know if it goes much higher as I usually turn it off.

I replaced the water pump and thermostat. When I remove the thermostat it runs cooler but the head near the spark plugs still is getting up to 160 deg range. When I remove the the thermostat cover and run the motor lots of water is pumping.

I am concerned because I do not see much water coming out of what I believe is the exhaust (half way down the back of the leg.
I see some a little 2 stroke smoke when it's cold and it does spit a bit when it warms up but not the amount I would expect. This leg doesn't look like it exhausts through the prop.

Does the exhaust come out the back of the leg half way down like other motors ?
Should I expect to see more water or not alot ?
Am I being to concerned about nothing ?
 
BTW, the telltale has lots of water coming out of it. I did pull the head off and it was a bit goopy in there but didn't look completely plugged or anything. I will be cleaning the head and the passage ways tonight, bolting it back together and hoping that it was just restricted enough to affect the flow of water.
My biggest concern right now is where is all that water going if it's not coming out the back of the leg.
 
The water exits out the snout just above the prop !!-----Is the water 8" above the impeller in your test tank ?------The cylinder head is warped to some degree.----Refinish it to make it flat before installing with new head gasket.
 
racerone said:
The water exits out the snout just above the prop !!-----Is the water 8" above the impeller in your test tank ?------The cylinder head is warped to some degree.----Refinish it to make it flat before installing with new head gasket.
Click to expand...

Is this snout you speak of below the water line or is it the rectangular block half way up the leg ?
https://www.marineengine.com/newparts/part_search.php?part_num=325936

Also, should this spit water like a 9.9 or 6hp OMC ? They tend to have a fair bit of water coming out middle hold on the back of the exhaust housing. This one has very little coming out. I tried hitting that port with compressed air but didn't have much success.

Thanks for your advice on this.
 
racerone said:
The water exits out the snout just above the prop !!-----Is the water 8" above the impeller in your test tank ?------The cylinder head is warped to some degree.----Refinish it to make it flat before installing with new head gasket.
Click to expand...

So I pulled the head off, refinished it as Racerone suggested and cleaned out all of the passages. There was some goo in them but not terrible. No obvious major blockages. I put it all back together and ran it last night. After about 10 mins in the tank, the temp got as high as 176 degs on the the head directly left of the top spark plug. The bottom one was cooler.

Is this normal ? I did try it idling and in gear. Both trolling speed and in gear with some throttle. I also tested the thermostat and it opens around 155 deg.
That just seems too hot to me. I will
 
I thought I’d reach out one more time. I ran this at trolling speed for 20 mins in my tank and it ran OK. I took it out for a rip today on the water and it ran beautiful but I only went about a mile total. Should I be worried about the heat in the tank getting up as high as 180 deg ? It runs beautifully and idles down great. I just don’t want to have missed something that could cause issues later.
Thank you for your thoughts on this.
 
FishyBC said:
I thought I’d reach out one more time. I ran this at trolling speed for 20 mins in my tank and it ran OK. I took it out for a rip today on the water and it ran beautiful but I only went about a mile total. Should I be worried about the heat in the tank getting up as high as 180 deg ? It runs beautifully and idles down great. I just don’t want to have missed something that could cause issues later.
Thank you for your thoughts on this.
Click to expand...

Hello, I had a 6hp that had a head like yours. I am not exactly familiar with the 7.5hp, but it appears to be a semi water cooled hot head style, meaning part is water cooled, but the spark plug area is not. So it would be normal for that area to be more hot than others. Your tub water will indeed warm up. Mine does when i run my motors in a tub. Good that you cleaned out the water passages in the block and head. I have seen a quite a few all full of junk. Fixed a johnson that the head warped bad, .060" in the width. Pulled the head and bottom of the water jacket was full of sand. Yanked the powerhead, cleaned every small passage spotless, new pump and good as new. Head was hot like yours near the plug.
Wish I could get on the lake here. Foot of ice yet..
 
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You are sure it was not just 0.006"-----I doubt the head can warp by 1/16" over the width.-----If it was the motor would certainly have been scrap from water leaking into the cylinders !!!-----Have a few of these 7.5 / 8 hp models apart at tthis time.----Heads will be trued up from warpage but not 1/16".
 
Was truely .060 to flatten it out. We swapped plugs to chase the misfire and just hand threaded them in. It looked like a lawn sprayer so much water came around the plug. Surprised we never bent a rod. The old timer the motor came from ran it for years like that. Cooling jacket loaded with sand. Dont remember compression numbers, but remember it was poor after the repair. But motor ran, so was "good enough" the current customer.
 
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