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  1. #1

    Default 1974 15hp Johnson temperature problems.

    Hi there,

    I recently acquired a little 1974 15hp Johnson. After checking it all out and adding a few missing parts. I was ready to test in my barrel. Nothing worked first go. After a carb kit and points clean/adjust I had it running smoothly. It was however getting a little hot. Id already checked the water pump and it seemed to pump water ok. I pulled the thermo and tested it. It opened in hot water and closed back down when colder. It was opening and seemed ok. To be safe I ordered a new one anyway. The new one is a 143 degree F unit. As an experiment I tested both side by side with a piece of string suspended in a pot of water over a hotplate. I brought the water up to temperature slowly and found the new thermo opened much earlier than the old. So I believed this to be my problem. I installed the new thermo with a new water jacket gasket and run it again in my water barrel. No difference. This motor is getting too hot. With the thermo removed it runs cool.

    Could I have a blocked outlet passage somewhere? The new thermo seems to operate ok. The water pump seems to be ok. I can probably get away with running it without the thermo where I live but its there for a reason and there must be another issue if its getting hot with the thermo in place. Im going to run it next with the water cover off and thermo out to see how well the water pump is pumping. Im pretty sure that will be ok and Im wondering what to look at next.

    any ideas?

    cheers

  2. #2
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    15,244

    Default Re: 1974 15hp Johnson temperature problems.

    Is the water well above the pump on your test bucket ?

  3. #3
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    4,926

    Default Re: 1974 15hp Johnson temperature problems.

    Pull the thermostat and start it up water should immediately gush out the thermostat hole if not you have restriction or a bad pump? Are you sure the water tube is snug on the water pump?
    It takes a nickle to go first class!
    If it aint broke dont fix it!!

  4. #4
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 1974 15hp Johnson temperature problems.

    When you initially checked the water pump, how did you do that? Did you use a drill and run just the lower unit in water? When you are operating in the test barrel, does it overheat in neutral too?

  5. #5
    Join Date
    Feb 2001
    Location
    Central West Florida
    Posts
    14,507

    Default Re: 1974 15hp Johnson temperature problems.

    As "racerone" above suggests and states in not so many words...... the water pump is not self priming and needs to be submerged to a point whereas the water level is a couple inches or so "above" the water pump, otherwise the water pump will draw air.

    The thermostat opens at 143 degrees so the engine is going to run somewhat hotter than that... and that's pretty hot! However I assume that you know when hot is too hot.... prints removed from fingers, that sort of thing!

    I'd suggest you drop the lower unit to inspect the water pump components. If you have been keeping the water level in your bucket, tank, whatever, above the water pump location, it's quite possible that it's just a case of the impeller has taken a set (permanent curvature) of the blades. Drop the lower unit and eliminate the guess-work.
    We occasionally have questions. If you fail to answer, it may affect ours.

  6. #6
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 1974 15hp Johnson temperature problems.

    Is this a saltwater use motor?

  7. #7
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 1974 15hp Johnson temperature problems.

    Well......what happened? I am curious. I am involved in these forums to learn more.....not simply give advice, thanks.

  8. #8

    Default Re: 1974 15hp Johnson temperature problems.

    Hi guys,

    I wrote this long winded reply asking more questions and answering some and it seems to have gone. I hit the reply button but I dont see my message here.

    Anyway to keep you up to date, in a shorter version.

    My test bucket has my motor well covered and replicates being on the boat in the water. I dont really know the history of this motor as I havent had it long. Could be salt water and probably is/was.

    I tested the lower unit with a multi speed drill and it pumps plenty of water even in low speed.

    My next step is to run it without the thermo in place with the water cover off. I would expect a lot of water to be pumped out. If this tests ok then it must be on the outlet side.

    In theory if I have the thermo out there should be no restricition. Is that right? Could I then use a garden hose on the water tube at low pressure and see the water exiting where it should? Should I see water out the relief holes in the midsection?

    Sorry I havent progressed much further as I had a busy weekend and didnt get back to it yet.

    cheers

  9. #9
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 1974 15hp Johnson temperature problems.

    You could start it up with some muffs and see how much water shoots out the prop. Could be that is is partially plugged up with oxidation and deposits from salt water. The little wife of mine has a place on the ocean in Southern Philippines. Jet Skis only last 3 or 4 years. Boats rarely have outboards, 99 percent of boats are straight inboards with an air cooling, or closed freshwater system. Worked on a pretty IO that had a 305 Chev. The owner religiously flushed it immediately upon trailering home. Most can't afford this factory made type boat......very expensive to import. So they make their own boats in the Philippines. Beautiful, beautiful.......using small truck diesels. I rode on a 100 passenger, made of wood, that was over 20 years old, run 365 days a year and 366 days on leap year.
    Last edited by timguy; 08-19-2018 at 11:37 PM.

  10. #10
    Join Date
    Nov 2005
    Location
    Mt. Zion, Illinois,
    Posts
    737

    Default Re: 1974 15hp Johnson temperature problems.

    As others said, if you don't know the age of the water pump for certain, replace it. If that's not the issue try these two options.

    Step 1, you could remove the exhaust cover plate and inspect your passages for blockages. You'll need to remove #'s 23 and 24 in this diagram and will require 2 new gaskets part #318925 to reassemble. Since you said it's possible this motor has been in salt water there's a chance you have junk built up under the plate.
    http://www.marineengine.com/parts/jo...+And+Crankcase

    If that doesn't work, then Step 2 is to remove the power head. In the exhaust housing diagram you'll see part #39. Joe Reeves put me onto this little guy when my '79 9.9 was having issues getting good water flow. It seems the upper water tube grommet has a habit of deforming and closing off the orifice. This will allow a little water flow so at low engine speed it seems to get water but at high speed it's not enough and the engine gets too hot.
    http://www.marineengine.com/parts/jo...xhaust+Housing

    For this work you'll need the grommet, part # 0324045, and a new base gasket, part # 0325721. When I pulled mine apart the opening in the grommet was closed down to only about the size of a pencil lead. The tell-tale only dribbled water when the t-stat opened and when I would go full throttle there was no water at all coming out of it.

  11. #11

    Default Re: 1974 15hp Johnson temperature problems.

    Thanks guys. I'm hoping to get some more time on it over the coming weekend. I'll just get some parts and pull it down. I'll get back to you with my findings. I might even have the gaskets I need. I've rebuilt one of these things in the past and had a bunch of extra gaskets etc.

    cheers

  12. #12
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 1974 15hp Johnson temperature problems.

    I have seen that grommet fail too, but you can check flow without removing the powerhead. I have taken a mouth full of water and blown into a 5/16 or 3/8 hose fastened to the copper supply tube going up to the powerhead. I know how much water flow by feel. We run these motors still at our resorts in NW Ontario. They are a great motor to have for customers with little or no outboard operating experience.

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