1978 Johnson 25 hp cooling issue? or not?

gaffemaster

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I believe that I am having some kind of cooling issue with my 25R78C. I will start the motor and it pumps water hard when its cold. I will then make a short run and bring it back down to idle and the tell tale will start to sputter and eventually steam comes out until you give it some throttle and then the water picks up again. It also seems that the exhaust below the water line stops bubbling out until you get on the throttle again. I just replaced the impeller, plate and housing in August of 2014. I have run it in a tank up to temp and it never seemed to lack on the water like I explained above but when out on the river it has done it every time this year so I'm not running it until I can figure out the issue for fear of over heating. I have read somewhere where guys say this was prone to overheating on the exhaust side. Is there anything that can remedy that problem? I have already redone the lower unit, coils and power pack, carb and fuel pump kit. I believe the motor is strong but am I sticking money into a time bomb? My next consideration was a new head gasket, thermostat and exhaust housing gaskets. All help is greatly appreciated.
 

fireman57

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You can try to take the thermostat out and run a piece of weedeater line through the ports and try to flush whatever blockage is there out. Take your lower unit off and hook a hose directly to the water intake tube and turn it on with the stat out. I have had to take two exhaust sides off already this year to clear blockages. A lot of people use a sealant when they replace an impellor and it gets loose and blocks these passages.
 

gaffemaster

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Thanks for the response. Does it sound like there could be a more serious issue or just a blockage in the water passages. I'd really like to keep the motor, as it seems to be easy to work on, but the more I seem to research this exact model the more hesitant I become investing money and time into it. It looks like it was well cared for and everything was original until I started rebuilding it. Compression was 115 on both cylinders when I bought it in February of 2012. I believe that was a wet test with the other plug in. Granted it is recoil start I got a dry reading the other day of just over 100 on both cylinders with plugs out. I don't mind spending a little more money if the motor reliability is there but is there something more I should be considering before making another investment into this particular motor. Just to clarify when the wet test was done the motor was hooked up to the gas line and I am not sure if the motor was warm. When the dry test was done the motor was cold if that makes any difference as far as compression readings go. Thanks.
 
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fireman57

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You can find something bad about every engine if you read enough. This thing is fine but won't be if it overheats.
 

gaffemaster

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I just can't seem to figure out why it would pump water strong out of the tell tale and bubble exhaust under water as well as exhaust ports on the housing when cold but after a run and getting hot they both will lack until rpms are higher. Also when I bring back down to idle it does seem to die and have a harder time starting when hot so that leads me to believe the head gasket is probably getting worn. This is my first outboard if that makes any difference so some things are still unfamiliar to me. They seem to be strong motors but I want to make sure I am maintaining it properly so it will last another lifetime. Thank you.
 

fireman57

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Changing the head gasket on that engine is not a big deal. Just make sure that you torque the bolts in the right order. Run it for a couple hours and retorque them. I am still leaning toward a blockage. When it is running wide open you get water restriction and then steam when you slow down. I would be if you let it idle longer the stream would eventually come back but I wouldn't trust blowing up an engine when you can fix it. Take your time. If you do end up removing the exhaust side bolts make sure you ratchet them back and forth a little instead of just trying to take them out. They are prone to breaking off from the heat and corrosion that seems to build there.
 

gaffemaster

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The reason I was looking to replace the exhaust housing gaskets is because it has a slow leak from the bottom center bolt. What else would you recommend looking over? From what I know everything is original on the motor besides what work I have already stated. The inside looks clean and none of the bolts are rusting. Is there any other seals that fail or need replacing over time besides the work I am considering which is head gasket, thermostat and gaskets as well as the exhaust housing gaskets? I appreciate you taking the time to share your knowledge with me. Thank you sir.
 

bonz_d

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I have owned one of those engines and they are just fine. My younger brother now owns it. I now have 1993 30HP, basically the same engine.
When was the last time the water pump was changed or rebuilt? I don't see any reason to be changing a head gasket in this case. Compression seems normal and you are not leaking water into a cylinder. Head Gaskets do not wear.

If it's getting hot then yes it will be harder to start and for a number of reasons. Where is the leak?
 

fireman57

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bonz, he said he changed impellor in Aug of 2014. Unless the little pin wasn't seated right, or half moon whichever he had, it should still be fine. I have had to change two exhaust sides this year already. Seems for some reason calcium builds up in those small passages.
 

bonz_d

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Still think I'd be pulling that lower unit down, doesn't take long to do it and it wouldn't be the 1st time a pump had premature failure. A quick visual inspection cost nothing but time.
 

gaffemaster

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Thank you guys for your help. fireman57 I am a little confused on how and where to run this weed eater line. Could explain the steps I need to take in more detail and what I should be looking for? Would a leak down test tell me anything more about the more I don't know and is it even necessary? I really appreciate all your help guys. Thanks again.
 

fireman57

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Take your thermostat out and run the line through the water passages, The holes under the thermostat. If need to find them go to ishopmarine.com and you can pull up your engine diagram there. You will probably need to drop the lower unit and flush it with a hose back through the engine with the tstats out.
 

gaffemaster

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Thanks for all the help guys. Picked up a Clymers manual at the library and will be trying to diagnose the problem this weekend. Appreciate all the help once again.
 

gaffemaster

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Just an update. Ended up pulling the head off and it turns out the head gasket was shot. The rings of the head gasket were not cracked but the gasket material covering the cooling jacket on the top cylinder was blown out. Water circulated fine until the motor would get warm and then the gases started escaping into the water jacket causing a pressure block is my guess. Perhaps it was exhaust coming from the tell tale and not steam? Pulled the exhaust cover and cleaned all the carbon build up on the plate as well. Plate had a nice amount of build up on it. New exhaust gaskets and head gasket with a resurfaced head and this thing is running like a dream. Hopefully my experience can help others. I appreciate all the help I received from you guys. Great community here. Thanks again
 

fireman57

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Just glad the bolts on the exhaust side came out nicely. They can cause big problems. Make sure you retorque the head after running it.
 
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