Evinrude 1993 50hp Motor Overheating.

[rajabatis]

I have had this motor for a year or so on my boat without any problems in fact used it on a fishing trip only a few days ago.
Today checking the motor out and running it on muffs. Pee pipe showing a good flow.
When I tried to turn off the motor it would not switch off and ran on at very high revs. I could not get it to stop.
The thermostat housing melted and popped out so did the temperature plug.
I can't figure what happened here. The tell tale was pumping out water okay and it seemed fine then I revved it up a bit to test the throttle was okay and it would not stop. Throttled back nothing! Switched off nothing! It just kept going. I eventually choked the carb to stop it.
Once it had cooled down I got it started again but with water now coming out of the thermostat housing! The rest of the motor seems fine. Runs okay sounds okay after I plugged up the thermostat hole to keep the water in!
Any ideas guys what has happened here?
Thanks.

 

[tashasdaddy]

Re: Evinrude 1993 50hp Motor Overheating.

you had a runaway engine, never run an engine on muffs above 1500 rpms. you are lucky if thats the only damage done, if it is. have to wait and see. check compression, and look for any burnt wires.

 

[rajabatis]

Re: Evinrude 1993 50hp Motor Overheating.

Thanks for the reply. Runaway? Yes I guess that's a good description of it. But why did it happen? I didn't know that was a problem when running over those suggested revs. Why is it that you can't run a motor on muffs over those revs? I've ran it up and down the throttle lower to mid range before without any problems before when flushing it after sea use.
I don't have the equipment to test the compression. I'll get that sorted out asap. I have had a good look round and these parts externally at least seem to be all that's damaged. I checked the plugs. They are okay and the motor when I plug the thermostat hole is starting and running okay.
I can't get over how quickly that engine went crazy though!

 

[tashasdaddy]

Re: Evinrude 1993 50hp Motor Overheating.

it's call runaway because that what it is. when on muffs there is no backpressure on the exhaust, and no load on the engine, so when it gets warm it, wants to do what it was made to do. there are cases where people pull the plug wires off and the keep running, like a diesel engine, you have to remove fuel in order to kill it. before it blows up. as i said you got lucky.

when flushing you don't need to rev the engine, all you want to do is bring the engine up to temperature, so the thermostats open, and circulate the water thru the engine.

 

[tashasdaddy]

Re: Evinrude 1993 50hp Motor Overheating.

i reposted your thread for the education of others on the forum.
http://forums.iboats.com/showthread.php?t=195016