Overheating?

[rp3703]

Hi,

I have a early 80’s Mariner(rebranded Yamaha) 25C 695 that I rebuilt. Motor runs fine now but I have only run it in a 50 Gal drum, my boat is in another location. The issue I’m having which may not even be an issue, is that the power head seemed really hot, way too hot to touch, after running it for 5-10 min or so. Again, it was running in a 50 Gal drum, so it was just recycling the same water, over and over again. The stream was good and strong and I could see that the thermostat was cutting it on and off as I cranked the throttle. Should I be concerned?

I should probably state that the reason I rebuilt the motor was because the second time I put it on the water after buying it used, it ran about a 1/4 mile and then started to lose power to the point of not running at all. I assumed it was because the cooling system failed and the engine was locking up but never confirmed. While rebuilding the motor, it turned out that one of the secondary coils was shot, so it could have been that the coil failed while running and the motor was only running on one cylinder. Again, this is all a guess. The motor seemed to run fine when I ran it behind a boat the first time but it did hiss after a while when it idled down but I have a feeling this was due to not having any thermostat installed.

I say I rebuilt the motor but I did not mess with the lower end(shifts good so did not see any reason to), did not remove the power head from the midsection(did not want to have to purchase and replace exhaust gasket between, again saw no reason to ) and did not rebuild power head itself(compression was above 100). I did replace the entire water pump with new OEM pump. Also replaced the seal between pump and lower end which was cracked and held together with JB Weld and gasket maker. I replaced the bad secondary coil. I pulled the head and replaced the head gasket. Installed a thermostat(did not have one at all). Replaced fuel pump and fuel filter. Drained and replaced the lower end oil. Cleaned the carb and replaced a cracked float(need a new float needle but have not found one yet). Set the points. Set the throttle. Then to top it all off, I added electric start.

 

[rp3703]

Hi,

I have a early 80’s Mariner(rebranded Yamaha) 25C 695 that I rebuilt. Motor runs fine now but I have only run it in a 50 Gal drum, my boat is in another location. The issue I’m having which may not even be an issue, is that the power head seemed really hot, way too hot to touch, after running it for 5-10 min or so. Again, it was running in a 50 Gal drum, so it was just recycling the same water, over and over again. The stream was good and strong and I could see that the thermostat was cutting it on and off as I cranked the throttle. Should I be concerned?

I should probably state that the reason I rebuilt the motor was because the second time I put it on the water after buying it used, it ran about a 1/4 mile and then started to lose power to the point of not running at all. I assumed it was because the cooling system failed and the engine was locking up but never confirmed. While rebuilding the motor, it turned out that one of the secondary coils was shot, so it could have been that the coil failed while running and the motor was only running on one cylinder. Again, this is all a guess. The motor seemed to run fine when I ran it behind a boat the first time but it did hiss after a while when it idled down but I have a feeling this was due to not having any thermostat installed.

I say I rebuilt the motor but I did not mess with the lower end(shifts good so did not see any reason to), did not remove the power head from the midsection(did not want to have to purchase and replace exhaust gasket between, again saw no reason to ) and did not rebuild power head itself(compression was above 100). I did replace the entire water pump with new OEM pump. Also replaced the seal between pump and lower end which was cracked and held together with JB Weld and gasket maker. I replaced the bad secondary coil. I pulled the head and replaced the head gasket. Installed a thermostat(did not have one at all). Replaced fuel pump and fuel filter. Drained and replaced the lower end oil. Cleaned the carb and replaced a cracked float(need a new float needle but have not found one yet). Set the points. Set the throttle. Then to top it all off, I added electric start.

 

[Chris1956]

If the powerhead is hot, she is overheating. A 50 Gal drum of water will cool that motor for hours....

Did you clean the carb? Lean running can cause overheat. Also a leaking head gasket can cause overheat.

BTW - I would not call your maintenance a "rebuiild".

 

[rp3703]

Yes, the carb was cleaned but the inlet needle looked like it may have some wear on the sides. It was not perfectly cone shaped. I would think this would cause the fuel mix to be more rich than lean. Also, I have only been able to set idle mix screw while running motor in 50 Gal drum. I have read that this should be set with motor pushing a boat.

 

[rp3703]

Head gasket was replaced as well along with every other gasket I could find that was missing.

 

[rp3703]

Update. So I replaced the needle valve and seat in the carb thinking it may be a fuel issue. It still runs hot. My temperature gauge touching the head read between 160-170 at idle. I have a Johnson 25 that I just rebuilt and it was reading 130 running in a tank at idle. The only other thing I can think of is to remove the exhaust manifold to see if there's any garbage in the cooling channel but they don't make new exhaust manifold gaskets for this motor anymore and I can't find NOS anywhere. Can these be made from blank gasket material? Can the old ones be reused?

 

[Texasmark]

The water pump has to be submerged as that centrifugal pump has no lifting capability....if you don't believe me and yours is above your water line in the tank, submerge it a couple of inches and with cool water in the drum watch your OT problem disappear....if that was your original problem.

When young I ran a 10 hp Scott-Attwater in a 55 gallon drum many, many times and it wouldn't take very long for the water in the drum to get hot if I didn't have the hose running in it while running the engine.

On hot, on My last Merc, thermostat started opening at 143 F. I think some smaller engines open at 125 or something thereabouts......I think I recall others mentioning that temp on them. The OT alarm on my last engine was activated at 192F. For a reference, the US domestic hot water heater in the home set to N is 140F. For a personal reference, at 140-150 if you touch the block at that temp you can keep your finger on it for about 2 seconds. If you have a non conducting infrared thermometer (Harbor Freight has them for a song) shoot the block and get a temp.

 

[rp3703]

I had this motor running in 55 gallon tank with new water from hose but hose was turned off. Water in tank was mid way up midsection of motor, well over water pump height. Water in tank was cool not warm. Water coming out pee stream was cool not warm or hot. The pee stream on my Johnson 25 was warm but head was cooler.

I used the temperature attachment on my Fluke multimeter to take temp readings. I would guess it is as accurate as infrared thermometer.

When I pulled head to replace head gasket, the water channels around cylinders were clean. I assumed the rest of the water circulation channels on powerhead were clean as well, especially since pee stream was good.

So do you think 160-170 is too hot? It is at idle. Again, everything in water pump has been replaced and motor has never been run dry or where pump was not submerged enough. I have never removed exhaust manifold and looked inside.

 

[rp3703]

Update, I pulled the exhaust manifold. Everything looked fine. I cleaned it all up and hand cut 2 new gaskets, since they no longer make them for this motor and put it back together. I guess 160-170 is just the normal idling temperature for this motor.

 

[Texasmark]

A fully opened T stat would be in that range, starting at 143 for opening temp (usually stamped on the pellet of the stat), 10-15 degrees to full open sounds normal. With your OT horn set to 195 or 192 forget which, you are a long way from Mercury deciding that you are running too hot. My guess......close her up and go out and go boating. Grin.

 

[rp3703]

Thanks Mark.