Mercury 8HP 2 stroke ouboard cooling issues

[rosinaboat]

I have inherited an outboard dating from 2000 but has had very little use. Since I got it it has had an intermittent amount of water going through it causing rapid overheating. I stripped it down and replaced the impeller and all gaskets. Now it has a beautiful stream at idle or slightly higher revs, but as soon as any power is applied the stream stops, no water gets into it and it overheats very quickly.

Has anyone experienced this? its driving me mad. I can see no reason why the impeller would slip on its shaft at higher revs.

Any thought very gratefully recieved, thanks Jonathan /forums/images/graemlins/confused.gif

 

[earlybird]

Are you, by any chance, testing the engine in a water barrel rather than on a boat? If so, the prop. should be removed otherwise extreme turbulence can give the effect you describe. Otherwise can't think of a possible reason.

 

[VicS]

I agree with earlybird. Even the exhaust gases can be a problem in a small water reservoir.

If we are not talking about testing in small barrel then I too am at a loss.

If that's not the problem and no other ideas are forthcoming try putting the question in the Mercury/Mariner forum at iBoats

 

[landlockedpirate]

I'm sure this model had a thermostat which controls the flow of cooling water according to conditions. Have a look at it and the switching mechanism.

 

[VicS]

[ QUOTE ]
I'm sure this model had a thermostat

[/ QUOTE ] It does and it's under a small cover at the very top of the cylinder head although I don't see how its malfunctioning would produce the symptoms described. Not sure I know what you mean by "switching mechanism" either.
However if all else fails it could be tested in a pan of water to see if it opens at the correct temperature.

 

[landlockedpirate]

My dad had a similar model, water would stop and start according to how hot the engine was. I thought that if this was malfunctioning it may cut off water flow when it should be fully open ?

 

[srp]

I have a Yamaha 5hp which had similar (but not the same) symptoms. From cold, it would start perfectly and happily run for ages on tickover. Opening the throttle would cause it to overheat and the tell-tale would turn from a gentle piddle to a wisp of steam. The thermostat was completely coked up with salt deposits. Taking it out and throwing it away cured the problem. I also used a bit of bent wire and a thin tube on a vacuum cleaner to clean out the cooling waterways as far as I could.

Edit - should read 'Re: rosinaboat '. Sorry Vic, sloppy posting by me!

 

[rosinaboat]

Thank you so much for your thoughts, you have given me two avenues to explore. The thermostat issue sounds like the most obvious cause, but I have always had a sneaking suspicion that the motor is mounted too high (on the transom of a Farr 6).

I really appreciate all your assistance, Jonathan

 

[VicS]

[ QUOTE ]
but I have always had a sneaking suspicion that the motor is mounted too high (on the transom of a Farr 6).

[/ QUOTE ] We are therefore talking about the symptoms you describe while it is in use on the boat and not as originally assumed in a barrel.

You have worked on the water pump so you know where it is located. In use that must be below the water level, if it is all should be well from that point of view.


If you measure from the top of the transom or mounting bracket to the waterline that will give you an indication of the required shaft length. Short (sometimes described as standard) shaft engines are 15" (381mm), long-shafts 20" (508mm) and extra long-shafts, where they exist, 25" (635mm).