81 Johnson 7.5HP Overheating

[kster]

Hello, I'm hoping for some help troubleshooting my 1981 7.5 HP Johnson seahorse . I just bought the motor thru auction. It was supposed to be reconditioned and ready for boating use. Well, it started nice and ran well but I am not familiar with these outboards much and did not know if this little motor had a water discharge off the back of the powerhead so I didnt think much of the fact I didnt see any at first. After a few minutes of run I noticed some water comming out and figured it just took a little while for it to pump up. I ran it for about 15 minutes before I really noticed the water was not flowing out consistant and not much at all at lower RPM. Then I felt the water discharge and it was very warm and shaft was hot and motor cover was hot! the lake water is still VERY cold here (39 degrees tops). I shut it down to let cool with cover off and had to run it again (15 min.) to get back home (there was at least some water flow). I think many people would say check the water pump right? Well, I do have a repair manual and am trying to study that procedure but I'd be thankful for any other input about this problem first. Does this motor have a thermostat to check on too? Do you think I did much damage to it? Where would an expert start? Hey, Thanks for any help! -Kster

 

[jtexas]

Re: 81 Johnson 7.5HP Overheating

welcome to the forum!

15 minutes running overtemp is pretty bad but it didn't sieze, that's a good sign. Not sure about an expert, but I'd start with a compression test - if the cyl aren't within 5% of each other pull the head & take a look.

Engine idling, you should be able to hold your fingers against the cylinder head for several seconds without permanent scars. Or much discomfort.

If the water pump is pumping, you should see a pretty good stream coming out the indicator at all speeds.

Replace the water pump and check the thermostat. Your manual should tell you at what temp it's supposed to open, you can test it with a cooking thermometer in boiling water.

 

[kster]

Re: 81 Johnson 7.5HP Overheating

Thank You much for your input Jtexas, I have a friend of a friend that I hope will help me with this motor this comming weekend. He is a outboard- Motorhead, even has a tatoo of a boat motor on him, I am told. I will try to check the comp. myself this week and start there. If I can't get this other guys help, I may have to try here again if I get stuck. This manual I have is not as clear about some things as I would like. Thanks again, it's cool that you people take some time to help the less experienced. I'v read some good stuff here already but not much on this 7.5 HP. I paid a few hundred for it and I hope it's not junk, I guess I'll find out. --Kster

 

[jauguston]

Re: 81 Johnson 7.5HP Overheating

Does the engine have a history of use in salt water? Salt corrosion can restrict the water passages.

 

[mchin]

Re: 81 Johnson 7.5HP Overheating

mineral laden water as well as fine silt and sand will clog up the ports of most any motor. Sometimes flushing works, but most times ya gotta scrape the scales off inside the water jacket. Especially if you've been running hot and the water inside has been boiling off. Like mentioned, check the thermostat, if it's crudded up or stuck, chances are the same stuff is choking off the water jackets.

I hope you stay with it, I like these motors and find them easy to work on and reliable when cared for.

 

[kster]

Re: 81 Johnson 7.5HP Overheating

Thanks guys, I don't know for sure but there is NO SIGN of salt water use that I can see from the outside but now I will have to get inside it and see, this is unfamiliar to me. The motor seems to be in great shape for it's age. There is very silty, sandy and grud in the waters up here now with spring runoff etc. I'm thinking maybe there is some junk caught up in the water passages because it still spits water but not enough. yes, the water was boiling off, I heard some sizzling. I'll check the comp. (hope the motor isnt cooked) and I have a water pump kit ordered. If comp. is good, I'll change the water pump, check thermostat, and flush system out and scrape if needed. Gosh, I hope I can get some help for this first time at least. For this motor, The comp. should be above 80 and less than 5 difference between the two? Right? Thanks, --Kster

 

[jauguston]

Re: 81 Johnson 7.5HP Overheating

If the water flow is still bad after you replace the impeller it would be a good idea to remove the cylinder head. You will be able to see the water passages that could be plugged. I did a 3.5 Nissan recently that was in salt most of its life and the passages were full of corrosion. A half hour with a Dremel tool with a 1/8" round nose rotary file and it was clean as new and cooled fine.

Jim

 

[jtexas]

Re: 81 Johnson 7.5HP Overheating

I don't know who that Dremel guy is, but his birthday ought to be a national holiday!

 

[kster]

Re: 81 Johnson 7.5HP Overheating

Hey everyone! I want to thank you all for your fast responces to my problem here. I am happy to inform you that I did accomplish this project on my own!!! Cleaned the gearcase up, it was full of sand & crud. Installed new water pump kit. Put everything back together. Pulled and checked and cleanded thermostat and then flushed the motor with the muffs. I just took it out today for a good run, it seemed to stay cool and it ran and "pee'd" water with force!!! Thanks again for your support !!!!!!!