The overheat alarm system question

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johnyrude200
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The overheat alarm system question

Postby johnyrude200 » Thu Aug 09, 2018 11:23 am

I recently had a personal motor, a 70hp seize up after being run a few hours. I used the motor for the last couple of years without issue.

There was no overheat alarm and it did not go into SLOW mode. I was at WOT, had slowed down to stop, and the motor just locked up.

Is it possible for this to happen so fast the sensor doesnt catch it?

Granted, it could have been a faulty sensor, but the motor pumps water like a fire hose and I actually ran it for another 3 hours that same day after it freed up. I pulled the thermostat to be safe but saw no evidence of any clogs or t-stat malfunction (new 2 years ago).

I had to use the motor to get back so had no choice. Top cylinder was badly scored yet cold compression numbers came back unaffected the next day. Motor has been since retired. When it warms up it’s very obvious the top cylinder is weak, and the motor is now clanging away at idle.
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FrankR
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Re: The overheat alarm system question

Postby FrankR » Thu Aug 09, 2018 12:26 pm

So,was it actually overheated (smoke, burnt paint)? Sounds more like a piston failure from some other cause.

But yes, I've seen lots of motors suddenly overheat from total and instant cooling failure at high speed, like impeller hub separation or plastic bag. They burned the paint and melted wires, and the alarm never went off. It all happened so quickly that the sensor didn't react till it was too late.

johnyrude200
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Re: The overheat alarm system question

Postby johnyrude200 » Thu Aug 09, 2018 2:05 pm

No smoke, no melted paint, in fact without pulling the head one might not even know anything happened unless turning over by hand when warm. Then it’s really obvious. Seemed a bit warm to the touch when I grabbed the flywheel but not any worst then what I would expect normally.

I suspect this was a pretty high hour motor because the compression wasnt great based on all the others Ive worked on. And I run my motors WOT all day long distances so it’s not like I baby them.

They’re cared for, but are put to work as well!
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FrankR
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Re: The overheat alarm system question

Postby FrankR » Thu Aug 09, 2018 4:07 pm

Time for an autopsy.

BillW
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Re: The overheat alarm system question

Postby BillW » Thu Aug 09, 2018 7:09 pm

Obvious, being a motor guy, you probably pulled the cover off right then. If it was overheated, believe me, you would smell it, feel the heat and probably still see a little smoke from the paint. I'm with Frank. I think you lost a piston. 70s were good motors, compared to the V engines, when it came to pistons; but they would still lunch one here and there. Especially if you're using the VRO system. Also, don't forget that lean running will fry up a piston. Maybe you had low fuel pressure for whatever reason or crud in the top carb.

bobw
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Re: The overheat alarm system question

Postby bobw » Thu Aug 09, 2018 7:24 pm

Same exact thing that happened to my Johnson 70 years ago. Running near wide open when the #1 piston let go. Post mortem found dirt in the top carb - Ran lean, got hot real quick and bang with no alarm.
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1954 Johnson CD-11
1958 Johnson QD-19
1972 Evinrude 25202

johnyrude200
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Re: The overheat alarm system question

Postby johnyrude200 » Thu Aug 09, 2018 8:59 pm

I appreciate everyone’s experience. Same conclusion I was drawing, lean on the top cylinder. I always run 40:1 and am religious when it comes to the cooling system. I think air came in through the crank or the top carb had a wallowed out plastic low spoed circuit/needle.
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