1979 Mercury 4cy 50 horse

[jessepeanut03]

I have a 1979 Mercury 50 horse 4 cy. I took it out on the local lake today and after about ten minutes at wot it shut off and was locked up. Couldn't turn it by hand or the starter. After getting home i took the spark plugs out and managed to force the eningine to turn over. It started back up and run fine. It didn't have any type of taping or knocking noises. I can't imagine how a motor would lock up and then start up and run fine. I'm completely lost. Any help would be much appreciated.

 

[racerone]

Start with removing the lower unit and inspect the water pump.---Many folks think they last forever .----Impeller should be replaced about every 3 years.

 

[jessepeanut03]

So your thinking maybe it overheated? I'm going to do a compression check this weekend and if all is good I had planned on replacing impeller and inspection all the water tubes inlets thermostat and ect. I'm still just curious as to how an engine could lock up then unlock and run fine

 

[Texasmark]

Sounds like over heating to me too. I had an Evinrude 70 hp 3 cyl that I bought used and was at least 10 years old. It operated fine initially on an outing, but as motor time increased it just didn't seem right. It had a thermostat and no water circulation tell tale, I was new to the engine, somewhat young and naive, etc.

One day at wide open throttle (WOT) I was blasting along and it seized up tight. In short the thermostat was stuck shut, the engine overheated at a high enough rpm that it did major damage before it shut itself down.

So, yes it can do what your's is doing and do it repeatedly until it gets bad enough enough and under the right conditions. Mine did.

That's probably the classic Merc. 4 cyl engine and they didn't have a thermostat, so that's out. However, racer is right on with the pump as I see it.

Mark

 

[jessepeanut03]

Thanks, makes sense. I hope all is good with the power head or my boating is done for a while. I won't be running it again until I have the issue taken care of. As good as it ran after it I got home I'm feel confident that nothing is damaged on the power head.

 

[Texasmark]

Good luck and when you get ready to do the work, get back on here and look in the archives on how to drop the lower unit on an engine that's 36 years young. Corrosion of the mounting bolts and nuts and getting the drive shaft and shift shafts unstuck will be your problems.

Mark

 

[jessepeanut03]

The power head is still good I dropped the lower unit. As easy as it was I would think it hasn't been long done. The impellar looked new with minimal wear but was quite rubbery. I'm not sure if it supposed to be soft or not. I thought maybe it was in with the hooks backwards. Does anyone know the way it is actually supposed to be in there? I'm going to order a new impellar. The water isn't coming out of the tell tale very strong and I figure it wouldn't hurt to buy a new one. It would be helpful to install the new one in the correct direction though.

 

[racerone]

The vanes on a new impeller will be straight !!!!!

 

[jessepeanut03]

They have the be bent to fit in the spacer in which direction should they be bent or will the bend the way they need to after the motor is turned over

 

[Texasmark]

Put some dishwashing liquid on the vanes and with the lid on top of the impeller, put slight/moderate pressure on the lid while rotating the driveshaft CW. The vanes will drop in the large area of the pump and pressure on the lid will keep them from popping out in the confined area till you get all seated and can push the cover down and seat it.

When running the ram water pressure from the water pickup and the speed of the drive shaft cause the vanes to fold back and the pump goes from a quasi displacement pump to a centrifugal pump......so I'm told, never looked inside one when running across the water at 40 mph to see what it looks like, but it makes sense.

Mark

 

[racerone]

Correct,---when running at slow speed the vanes flex and it is a displacement pump.--At higher speeds it becomes a centrifugal pump.----When vanes are permanently curved the pump will not function properly !!!!

 

[jessepeanut03]

So which direction should I turn the drive shaft. Clock ways counter clock ways or does it matter

 

[racerone]

When you push the pump housing down you rotate the driveshaft the way the motor turns it.---And yes it does matter.